diff --git a/packages/jest-react/src/internalAct.js b/packages/jest-react/src/internalAct.js
index d078c888c75ed..1b5349554c40d 100644
--- a/packages/jest-react/src/internalAct.js
+++ b/packages/jest-react/src/internalAct.js
@@ -122,7 +122,7 @@ export function act<T>(scope: () => Thenable<T> | T): Thenable<T> {
   }
 }
 
-function flushActWork(resolve, reject) {
+function flushActWork(resolve: () => void, reject: (error: any) => void) {
   if (Scheduler.unstable_hasPendingWork()) {
     try {
       Scheduler.unstable_flushUntilNextPaint();
diff --git a/packages/react-cache/src/ReactCacheOld.js b/packages/react-cache/src/ReactCacheOld.js
index 3c83b76cacbcd..99716076984c0 100644
--- a/packages/react-cache/src/ReactCacheOld.js
+++ b/packages/react-cache/src/ReactCacheOld.js
@@ -7,7 +7,7 @@
  * @flow
  */
 
-import type {Thenable} from 'shared/ReactTypes';
+import type {ReactContext, Thenable} from 'shared/ReactTypes';
 
 import * as React from 'react';
 
@@ -48,7 +48,7 @@ const ReactCurrentDispatcher =
   React.__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED
     .ReactCurrentDispatcher;
 
-function readContext(Context) {
+function readContext(Context: ReactContext<mixed>) {
   const dispatcher = ReactCurrentDispatcher.current;
   if (dispatcher === null) {
     // This wasn't being minified but we're going to retire this package anyway.
@@ -62,6 +62,7 @@ function readContext(Context) {
   return dispatcher.readContext(Context);
 }
 
+// $FlowFixMe[missing-local-annot]
 function identityHashFn(input) {
   if (__DEV__) {
     if (
@@ -133,7 +134,7 @@ function accessResult<I, K, V>(
   }
 }
 
-function deleteEntry(resource, key) {
+function deleteEntry(resource: any, key: mixed) {
   const entriesForResource = entries.get(resource);
   if (entriesForResource !== undefined) {
     entriesForResource.delete(key);
diff --git a/packages/react-client/src/ReactFlightClient.js b/packages/react-client/src/ReactFlightClient.js
index c023a1feca632..3c9d26338b660 100644
--- a/packages/react-client/src/ReactFlightClient.js
+++ b/packages/react-client/src/ReactFlightClient.js
@@ -94,6 +94,7 @@ type SomeChunk<T> =
   | InitializedChunk<T>
   | ErroredChunk<T>;
 
+// $FlowFixMe[missing-this-annot]
 function Chunk(status: any, value: any, reason: any, response: Response) {
   this.status = status;
   this.value = value;
@@ -104,6 +105,7 @@ function Chunk(status: any, value: any, reason: any, response: Response) {
 Chunk.prototype = (Object.create(Promise.prototype): any);
 // TODO: This doesn't return a new Promise chain unlike the real .then
 Chunk.prototype.then = function<T>(
+  this: SomeChunk<T>,
   resolve: (value: T) => mixed,
   reject: (reason: mixed) => mixed,
 ) {
@@ -369,7 +371,11 @@ export function reportGlobalError(response: Response, error: Error): void {
   });
 }
 
-function createElement(type, key, props): React$Element<any> {
+function createElement(
+  type: mixed,
+  key: mixed,
+  props: mixed,
+): React$Element<any> {
   const element: any = {
     // This tag allows us to uniquely identify this as a React Element
     $$typeof: REACT_ELEMENT_TYPE,
@@ -446,6 +452,7 @@ function createModelResolver<T>(
       value: null,
     };
   }
+  // $FlowFixMe[missing-local-annot]
   return value => {
     parentObject[key] = value;
     blocked.deps--;
@@ -465,7 +472,7 @@ function createModelResolver<T>(
 }
 
 function createModelReject<T>(chunk: SomeChunk<T>) {
-  return error => triggerErrorOnChunk(chunk, error);
+  return (error: mixed) => triggerErrorOnChunk(chunk, error);
 }
 
 export function parseModelString(
diff --git a/packages/react-client/src/ReactFlightClientStream.js b/packages/react-client/src/ReactFlightClientStream.js
index c3f3c48b1a5bc..4709ca8085ddc 100644
--- a/packages/react-client/src/ReactFlightClientStream.js
+++ b/packages/react-client/src/ReactFlightClientStream.js
@@ -122,6 +122,7 @@ export function processBinaryChunk(
 }
 
 function createFromJSONCallback(response: Response) {
+  // $FlowFixMe[missing-this-annot]
   return function(key: string, value: JSONValue) {
     if (typeof value === 'string') {
       // We can't use .bind here because we need the "this" value.
diff --git a/packages/react-debug-tools/src/ReactDebugHooks.js b/packages/react-debug-tools/src/ReactDebugHooks.js
index 5971c38a97afe..fbe4dc2f360a4 100644
--- a/packages/react-debug-tools/src/ReactDebugHooks.js
+++ b/packages/react-debug-tools/src/ReactDebugHooks.js
@@ -350,7 +350,7 @@ const Dispatcher: DispatcherType = {
 // create a proxy to throw a custom error
 // in case future versions of React adds more hooks
 const DispatcherProxyHandler = {
-  get(target, prop) {
+  get(target: DispatcherType, prop: string) {
     if (target.hasOwnProperty(prop)) {
       return target[prop];
     }
@@ -404,7 +404,7 @@ export type HooksTree = Array<HooksNode>;
 
 let mostLikelyAncestorIndex = 0;
 
-function findSharedIndex(hookStack, rootStack, rootIndex) {
+function findSharedIndex(hookStack: any, rootStack: any, rootIndex: number) {
   const source = rootStack[rootIndex].source;
   hookSearch: for (let i = 0; i < hookStack.length; i++) {
     if (hookStack[i].source === source) {
@@ -425,7 +425,7 @@ function findSharedIndex(hookStack, rootStack, rootIndex) {
   return -1;
 }
 
-function findCommonAncestorIndex(rootStack, hookStack) {
+function findCommonAncestorIndex(rootStack: any, hookStack: any) {
   let rootIndex = findSharedIndex(
     hookStack,
     rootStack,
@@ -446,7 +446,7 @@ function findCommonAncestorIndex(rootStack, hookStack) {
   return -1;
 }
 
-function isReactWrapper(functionName, primitiveName) {
+function isReactWrapper(functionName: any, primitiveName: string) {
   if (!functionName) {
     return false;
   }
@@ -460,7 +460,7 @@ function isReactWrapper(functionName, primitiveName) {
   );
 }
 
-function findPrimitiveIndex(hookStack, hook) {
+function findPrimitiveIndex(hookStack: any, hook: HookLogEntry) {
   const stackCache = getPrimitiveStackCache();
   const primitiveStack = stackCache.get(hook.primitive);
   if (primitiveStack === undefined) {
@@ -488,7 +488,7 @@ function findPrimitiveIndex(hookStack, hook) {
   return -1;
 }
 
-function parseTrimmedStack(rootStack, hook) {
+function parseTrimmedStack(rootStack: any, hook: HookLogEntry) {
   // Get the stack trace between the primitive hook function and
   // the root function call. I.e. the stack frames of custom hooks.
   const hookStack = ErrorStackParser.parse(hook.stackError);
@@ -520,8 +520,8 @@ function parseCustomHookName(functionName: void | string): string {
 }
 
 function buildTree(
-  rootStack,
-  readHookLog,
+  rootStack: any,
+  readHookLog: Array<HookLogEntry>,
   includeHooksSource: boolean,
 ): HooksTree {
   const rootChildren = [];
@@ -764,7 +764,7 @@ function inspectHooksOfForwardRef<Props, Ref>(
   return buildTree(rootStack, readHookLog, includeHooksSource);
 }
 
-function resolveDefaultProps(Component, baseProps) {
+function resolveDefaultProps(Component: any, baseProps: any) {
   if (Component && Component.defaultProps) {
     // Resolve default props. Taken from ReactElement
     const props = assign({}, baseProps);
diff --git a/packages/react-devtools-core/src/backend.js b/packages/react-devtools-core/src/backend.js
index 70048a9de6cd1..f14ece0eefd19 100644
--- a/packages/react-devtools-core/src/backend.js
+++ b/packages/react-devtools-core/src/backend.js
@@ -44,7 +44,7 @@ const hook: ?DevToolsHook = window.__REACT_DEVTOOLS_GLOBAL_HOOK__;
 
 let savedComponentFilters: Array<ComponentFilter> = getDefaultComponentFilters();
 
-function debug(methodName: string, ...args) {
+function debug(methodName: string, ...args: Array<mixed>) {
   if (__DEBUG__) {
     console.log(
       `%c[core/backend] %c${methodName}`,
@@ -276,7 +276,7 @@ export function connectToDevTools(options: ?ConnectOptions) {
     scheduleRetry();
   }
 
-  function handleMessage(event) {
+  function handleMessage(event: MessageEvent) {
     let data;
     try {
       if (typeof event.data === 'string') {
diff --git a/packages/react-devtools-core/src/standalone.js b/packages/react-devtools-core/src/standalone.js
index 1b11a4710099c..22cf1677003e1 100644
--- a/packages/react-devtools-core/src/standalone.js
+++ b/packages/react-devtools-core/src/standalone.js
@@ -86,11 +86,12 @@ let bridge: FrontendBridge | null = null;
 let store: Store | null = null;
 let root = null;
 
-const log = (...args) => console.log('[React DevTools]', ...args);
-log.warn = (...args) => console.warn('[React DevTools]', ...args);
-log.error = (...args) => console.error('[React DevTools]', ...args);
+const log = (...args: Array<mixed>) => console.log('[React DevTools]', ...args);
+log.warn = (...args: Array<mixed>) => console.warn('[React DevTools]', ...args);
+log.error = (...args: Array<mixed>) =>
+  console.error('[React DevTools]', ...args);
 
-function debug(methodName: string, ...args) {
+function debug(methodName: string, ...args: Array<mixed>) {
   if (__DEBUG__) {
     console.log(
       `%c[core/standalone] %c${methodName}`,
@@ -166,6 +167,7 @@ function onDisconnected() {
   disconnectedCallback();
 }
 
+// $FlowFixMe[missing-local-annot]
 function onError({code, message}) {
   safeUnmount();
 
diff --git a/packages/react-devtools-extensions/src/backend.js b/packages/react-devtools-extensions/src/backend.js
index ad55ed6d50fdc..5c397c3ee2128 100644
--- a/packages/react-devtools-extensions/src/backend.js
+++ b/packages/react-devtools-extensions/src/backend.js
@@ -8,6 +8,7 @@
 
 let welcomeHasInitialized = false;
 
+// $FlowFixMe[missing-local-annot]
 function welcome(event) {
   if (
     event.source !== window ||
@@ -42,7 +43,7 @@ function welcome(event) {
 
 window.addEventListener('message', welcome);
 
-function setup(hook) {
+function setup(hook: any) {
   if (hook == null) {
     // DevTools didn't get injected into this page (maybe b'c of the contentType).
     return;
@@ -55,6 +56,7 @@ function setup(hook) {
 
   const bridge = new Bridge({
     listen(fn) {
+      // $FlowFixMe[missing-local-annot]
       const listener = event => {
         if (
           event.source !== window ||
diff --git a/packages/react-devtools-inline/src/backend.js b/packages/react-devtools-inline/src/backend.js
index 97d2368ac1696..659b6c8521c95 100644
--- a/packages/react-devtools-inline/src/backend.js
+++ b/packages/react-devtools-inline/src/backend.js
@@ -10,6 +10,7 @@ import type {BackendBridge} from 'react-devtools-shared/src/bridge';
 import type {Wall} from 'react-devtools-shared/src/types';
 
 function startActivation(contentWindow: any, bridge: BackendBridge) {
+  // $FlowFixMe[missing-local-annot]
   const onSavedPreferences = data => {
     // This is the only message we're listening for,
     // so it's safe to cleanup after we've received it.
@@ -96,6 +97,7 @@ export function createBridge(contentWindow: any, wall?: Wall): BackendBridge {
   if (wall == null) {
     wall = {
       listen(fn) {
+        // $FlowFixMe[missing-local-annot]
         const onMessage = ({data}) => {
           fn(data);
         };
diff --git a/packages/react-devtools-inline/src/frontend.js b/packages/react-devtools-inline/src/frontend.js
index 272fbba0b9265..018ce76c5df25 100644
--- a/packages/react-devtools-inline/src/frontend.js
+++ b/packages/react-devtools-inline/src/frontend.js
@@ -37,6 +37,7 @@ export function createBridge(contentWindow: any, wall?: Wall): FrontendBridge {
   if (wall == null) {
     wall = {
       listen(fn) {
+        // $FlowFixMe[missing-local-annot]
         const onMessage = ({data}) => {
           fn(data);
         };
diff --git a/packages/react-devtools-shared/src/backend/agent.js b/packages/react-devtools-shared/src/backend/agent.js
index 78812286bf211..0af2033cf955f 100644
--- a/packages/react-devtools-shared/src/backend/agent.js
+++ b/packages/react-devtools-shared/src/backend/agent.js
@@ -42,7 +42,7 @@ import type {ComponentFilter} from '../types';
 import {isSynchronousXHRSupported} from './utils';
 import type {BrowserTheme} from 'react-devtools-shared/src/devtools/views/DevTools';
 
-const debug = (methodName, ...args) => {
+const debug = (methodName: string, ...args: Array<string>) => {
   if (__DEBUG__) {
     console.log(
       `%cAgent %c${methodName}`,
diff --git a/packages/react-devtools-shared/src/backend/console.js b/packages/react-devtools-shared/src/backend/console.js
index 5753d910eba1d..61ead6d7c478d 100644
--- a/packages/react-devtools-shared/src/backend/console.js
+++ b/packages/react-devtools-shared/src/backend/console.js
@@ -197,6 +197,7 @@ export function patch({
           ? targetConsole[method].__REACT_DEVTOOLS_ORIGINAL_METHOD__
           : targetConsole[method]);
 
+        // $FlowFixMe[missing-local-annot]
         const overrideMethod = (...args) => {
           let shouldAppendWarningStack = false;
           if (method !== 'log') {
@@ -335,6 +336,7 @@ export function patchForStrictMode() {
           ? targetConsole[method].__REACT_DEVTOOLS_STRICT_MODE_ORIGINAL_METHOD__
           : targetConsole[method]);
 
+        // $FlowFixMe[missing-local-annot]
         const overrideMethod = (...args) => {
           if (!consoleSettingsRef.hideConsoleLogsInStrictMode) {
             // Dim the text color of the double logs if we're not
diff --git a/packages/react-devtools-shared/src/backend/legacy/renderer.js b/packages/react-devtools-shared/src/backend/legacy/renderer.js
index 8d02e853a4060..1b667cf122185 100644
--- a/packages/react-devtools-shared/src/backend/legacy/renderer.js
+++ b/packages/react-devtools-shared/src/backend/legacy/renderer.js
@@ -195,7 +195,7 @@ export function attach(
     return ((internalInstanceToIDMap.get(internalInstance): any): number);
   }
 
-  function areEqualArrays(a, b) {
+  function areEqualArrays(a: Array<any>, b: Array<any>) {
     if (a.length !== b.length) {
       return false;
     }
diff --git a/packages/react-devtools-shared/src/backend/legacy/utils.js b/packages/react-devtools-shared/src/backend/legacy/utils.js
index 581f8e9222d20..bc443c1dd164e 100644
--- a/packages/react-devtools-shared/src/backend/legacy/utils.js
+++ b/packages/react-devtools-shared/src/backend/legacy/utils.js
@@ -11,6 +11,7 @@ import type {InternalInstance} from './renderer';
 
 export function decorate(object: Object, attr: string, fn: Function): Function {
   const old = object[attr];
+  // $FlowFixMe[missing-this-annot] webpack config needs to be updated to allow `this` type annotations
   object[attr] = function(instance: InternalInstance) {
     return fn.call(this, old, arguments);
   };
@@ -34,6 +35,7 @@ export function restoreMany(source: Object, olds: Object): void {
   }
 }
 
+// $FlowFixMe[missing-this-annot] webpack config needs to be updated to allow `this` type annotations
 export function forceUpdate(instance: InternalInstance): void {
   if (typeof instance.forceUpdate === 'function') {
     instance.forceUpdate();
diff --git a/packages/react-devtools-shared/src/backend/profilingHooks.js b/packages/react-devtools-shared/src/backend/profilingHooks.js
index f83875b14d1a5..47d173c55c3c4 100644
--- a/packages/react-devtools-shared/src/backend/profilingHooks.js
+++ b/packages/react-devtools-shared/src/backend/profilingHooks.js
@@ -204,7 +204,7 @@ export function createProfilingHooks({
     }
   }
 
-  function markAndClear(markName) {
+  function markAndClear(markName: string) {
     // This method won't be called unless these functions are defined, so we can skip the extra typeof check.
     ((performanceTarget: any): Performance).mark(markName);
     ((performanceTarget: any): Performance).clearMarks(markName);
diff --git a/packages/react-devtools-shared/src/backend/renderer.js b/packages/react-devtools-shared/src/backend/renderer.js
index 68425faac8d7a..54f95a2fbea6a 100644
--- a/packages/react-devtools-shared/src/backend/renderer.js
+++ b/packages/react-devtools-shared/src/backend/renderer.js
@@ -2546,7 +2546,7 @@ export function attach(
     // We don't patch any methods so there is no cleanup.
   }
 
-  function rootSupportsProfiling(root) {
+  function rootSupportsProfiling(root: any) {
     if (root.memoizedInteractions != null) {
       // v16 builds include this field for the scheduler/tracing API.
       return true;
@@ -2610,7 +2610,7 @@ export function attach(
     }
   }
 
-  function getUpdatersList(root): Array<SerializedElement> | null {
+  function getUpdatersList(root: any): Array<SerializedElement> | null {
     return root.memoizedUpdaters != null
       ? Array.from(root.memoizedUpdaters)
           .filter(fiber => getFiberIDUnsafe(fiber) !== null)
@@ -2618,7 +2618,7 @@ export function attach(
       : null;
   }
 
-  function handleCommitFiberUnmount(fiber) {
+  function handleCommitFiberUnmount(fiber: any) {
     // If the untrackFiberSet already has the unmounted Fiber, this means we've already
     // recordedUnmount, so we don't need to do it again. If we don't do this, we might
     // end up double-deleting Fibers in some cases (like Legacy Suspense).
@@ -2630,7 +2630,7 @@ export function attach(
     }
   }
 
-  function handlePostCommitFiberRoot(root) {
+  function handlePostCommitFiberRoot(root: any) {
     if (isProfiling && rootSupportsProfiling(root)) {
       if (currentCommitProfilingMetadata !== null) {
         const {effectDuration, passiveEffectDuration} = getEffectDurations(
@@ -2644,7 +2644,7 @@ export function attach(
     }
   }
 
-  function handleCommitFiberRoot(root, priorityLevel) {
+  function handleCommitFiberRoot(root: any, priorityLevel: void | number) {
     const current = root.current;
     const alternate = current.alternate;
 
@@ -2805,18 +2805,18 @@ export function attach(
     }
   }
 
-  function getDisplayNameForFiberID(id) {
+  function getDisplayNameForFiberID(id: number) {
     const fiber = idToArbitraryFiberMap.get(id);
     return fiber != null ? getDisplayNameForFiber(((fiber: any): Fiber)) : null;
   }
 
-  function getFiberForNative(hostInstance) {
+  function getFiberForNative(hostInstance: NativeType) {
     return renderer.findFiberByHostInstance(hostInstance);
   }
 
   function getFiberIDForNative(
-    hostInstance,
-    findNearestUnfilteredAncestor = false,
+    hostInstance: NativeType,
+    findNearestUnfilteredAncestor: boolean = false,
   ) {
     let fiber = renderer.findFiberByHostInstance(hostInstance);
     if (fiber != null) {
@@ -2832,7 +2832,7 @@ export function attach(
 
   // This function is copied from React and should be kept in sync:
   // https://github.com/facebook/react/blob/main/packages/react-reconciler/src/ReactFiberTreeReflection.js
-  function assertIsMounted(fiber) {
+  function assertIsMounted(fiber: Fiber) {
     if (getNearestMountedFiber(fiber) !== fiber) {
       throw new Error('Unable to find node on an unmounted component.');
     }
@@ -3712,7 +3712,7 @@ export function attach(
     };
   }
 
-  function logElementToConsole(id) {
+  function logElementToConsole(id: number) {
     const result = isMostRecentlyInspectedElementCurrent(id)
       ? mostRecentlyInspectedElement
       : inspectElementRaw(id);
@@ -4149,7 +4149,7 @@ export function attach(
   // null (do nothing)
   const forceErrorForFiberIDs = new Map();
 
-  function shouldErrorFiberAccordingToMap(fiber) {
+  function shouldErrorFiberAccordingToMap(fiber: any) {
     if (typeof setErrorHandler !== 'function') {
       throw new Error(
         'Expected overrideError() to not get called for earlier React versions.',
@@ -4185,7 +4185,7 @@ export function attach(
     return status;
   }
 
-  function overrideError(id, forceError) {
+  function overrideError(id: number, forceError: boolean) {
     if (
       typeof setErrorHandler !== 'function' ||
       typeof scheduleUpdate !== 'function'
@@ -4214,12 +4214,12 @@ export function attach(
 
   const forceFallbackForSuspenseIDs = new Set();
 
-  function shouldSuspendFiberAccordingToSet(fiber) {
+  function shouldSuspendFiberAccordingToSet(fiber: any) {
     const maybeID = getFiberIDUnsafe(((fiber: any): Fiber));
     return maybeID !== null && forceFallbackForSuspenseIDs.has(maybeID);
   }
 
-  function overrideSuspense(id, forceFallback) {
+  function overrideSuspense(id: number, forceFallback: boolean) {
     if (
       typeof setSuspenseHandler !== 'function' ||
       typeof scheduleUpdate !== 'function'
@@ -4317,7 +4317,9 @@ export function attach(
     return true;
   }
 
-  function updateTrackedPathStateAfterMount(mightSiblingsBeOnTrackedPath) {
+  function updateTrackedPathStateAfterMount(
+    mightSiblingsBeOnTrackedPath: boolean,
+  ) {
     // updateTrackedPathStateBeforeMount() told us whether to match siblings.
     // Now that we're entering siblings, let's use that information.
     mightBeOnTrackedPath = mightSiblingsBeOnTrackedPath;
diff --git a/packages/react-devtools-shared/src/backend/views/Highlighter/Overlay.js b/packages/react-devtools-shared/src/backend/views/Highlighter/Overlay.js
index 5414f3ef0efe9..f19a335bf79d7 100644
--- a/packages/react-devtools-shared/src/backend/views/Highlighter/Overlay.js
+++ b/packages/react-devtools-shared/src/backend/views/Highlighter/Overlay.js
@@ -277,7 +277,11 @@ export default class Overlay {
   }
 }
 
-function findTipPos(dims, bounds, tipSize) {
+function findTipPos(
+  dims: Box,
+  bounds: Box,
+  tipSize: {height: number, width: number},
+) {
   const tipHeight = Math.max(tipSize.height, 20);
   const tipWidth = Math.max(tipSize.width, 60);
   const margin = 5;
@@ -314,7 +318,7 @@ function findTipPos(dims, bounds, tipSize) {
   };
 }
 
-function boxWrap(dims, what, node) {
+function boxWrap(dims: any, what: string, node: HTMLElement) {
   assign(node.style, {
     borderTopWidth: dims[what + 'Top'] + 'px',
     borderLeftWidth: dims[what + 'Left'] + 'px',
diff --git a/packages/react-devtools-shared/src/backend/views/Highlighter/index.js b/packages/react-devtools-shared/src/backend/views/Highlighter/index.js
index 6793e92e51dd8..dada3978e2e3a 100644
--- a/packages/react-devtools-shared/src/backend/views/Highlighter/index.js
+++ b/packages/react-devtools-shared/src/backend/views/Highlighter/index.js
@@ -38,7 +38,7 @@ export default function setupHighlighter(
     registerListenersOnWindow(window);
   }
 
-  function registerListenersOnWindow(window) {
+  function registerListenersOnWindow(window: any) {
     // This plug-in may run in non-DOM environments (e.g. React Native).
     if (window && typeof window.addEventListener === 'function') {
       window.addEventListener('click', onClick, true);
@@ -66,7 +66,7 @@ export default function setupHighlighter(
     iframesListeningTo = new Set();
   }
 
-  function removeListenersOnWindow(window) {
+  function removeListenersOnWindow(window: any) {
     // This plug-in may run in non-DOM environments (e.g. React Native).
     if (window && typeof window.removeEventListener === 'function') {
       window.removeEventListener('click', onClick, true);
diff --git a/packages/react-devtools-shared/src/devtools/ContextMenu/ContextMenu.js b/packages/react-devtools-shared/src/devtools/ContextMenu/ContextMenu.js
index a800b58fc37e5..3b4d2b6964965 100644
--- a/packages/react-devtools-shared/src/devtools/ContextMenu/ContextMenu.js
+++ b/packages/react-devtools-shared/src/devtools/ContextMenu/ContextMenu.js
@@ -81,7 +81,15 @@ export default function ContextMenu({children, id}: Props): React.Node {
   }, []);
 
   useEffect(() => {
-    const showMenuFn = ({data, pageX, pageY}) => {
+    const showMenuFn = ({
+      data,
+      pageX,
+      pageY,
+    }: {
+      data: any,
+      pageX: number,
+      pageY: number,
+    }) => {
       setState({data, isVisible: true, pageX, pageY});
     };
     const hideMenuFn = () => setState(HIDDEN_STATE);
@@ -96,6 +104,7 @@ export default function ContextMenu({children, id}: Props): React.Node {
     const menu = ((menuRef.current: any): HTMLElement);
     const container = containerRef.current;
     if (container !== null) {
+      // $FlowFixMe[missing-local-annot]
       const hideUnlessContains = event => {
         if (!menu.contains(event.target)) {
           hideMenu();
diff --git a/packages/react-devtools-shared/src/devtools/ContextMenu/ContextMenuItem.js b/packages/react-devtools-shared/src/devtools/ContextMenu/ContextMenuItem.js
index 3de6acef774b1..d634f5eeef286 100644
--- a/packages/react-devtools-shared/src/devtools/ContextMenu/ContextMenuItem.js
+++ b/packages/react-devtools-shared/src/devtools/ContextMenu/ContextMenuItem.js
@@ -28,7 +28,7 @@ export default function ContextMenuItem({
 }: Props): React.Node {
   const {hideMenu} = useContext<RegistryContextType>(RegistryContext);
 
-  const handleClick = event => {
+  const handleClick = (event: any) => {
     onClick();
     hideMenu();
   };
diff --git a/packages/react-devtools-shared/src/devtools/cache.js b/packages/react-devtools-shared/src/devtools/cache.js
index 6afbc64ada418..c8f62ba3853c3 100644
--- a/packages/react-devtools-shared/src/devtools/cache.js
+++ b/packages/react-devtools-shared/src/devtools/cache.js
@@ -7,7 +7,7 @@
  * @flow
  */
 
-import type {Thenable} from 'shared/ReactTypes';
+import type {ReactContext, Thenable} from 'shared/ReactTypes';
 
 import * as React from 'react';
 import {createContext} from 'react';
@@ -63,7 +63,7 @@ const Rejected = 2;
 const ReactCurrentDispatcher = (React: any)
   .__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED.ReactCurrentDispatcher;
 
-function readContext(Context) {
+function readContext(Context: ReactContext<null>) {
   const dispatcher = ReactCurrentDispatcher.current;
   if (dispatcher === null) {
     throw new Error(
diff --git a/packages/react-devtools-shared/src/devtools/store.js b/packages/react-devtools-shared/src/devtools/store.js
index 71d53c886e67b..fd9264eed5a2b 100644
--- a/packages/react-devtools-shared/src/devtools/store.js
+++ b/packages/react-devtools-shared/src/devtools/store.js
@@ -46,7 +46,7 @@ import type {
 } from 'react-devtools-shared/src/bridge';
 import UnsupportedBridgeOperationError from 'react-devtools-shared/src/UnsupportedBridgeOperationError';
 
-const debug = (methodName, ...args) => {
+const debug = (methodName: string, ...args: Array<string>) => {
   if (__DEBUG__) {
     console.log(
       `%cStore %c${methodName}`,
@@ -1146,7 +1146,7 @@ export default class Store extends EventEmitter<{
             debug(`Remove root ${id}`);
           }
 
-          const recursivelyDeleteElements = elementID => {
+          const recursivelyDeleteElements = (elementID: number) => {
             const element = this._idToElement.get(elementID);
             this._idToElement.delete(elementID);
             if (element) {
@@ -1431,7 +1431,7 @@ export default class Store extends EventEmitter<{
   // but the downstream errors they cause will be reported as bugs.
   // For example, https://github.com/facebook/react/issues/21402
   // Emitting an error event allows the ErrorBoundary to show the original error.
-  _throwAndEmitError(error: Error) {
+  _throwAndEmitError(error: Error): empty {
     this.emit('error', error);
 
     // Throwing is still valuable for local development
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/ComponentSearchInput.js b/packages/react-devtools-shared/src/devtools/views/Components/ComponentSearchInput.js
index 3085f0a2120ed..f84e29bf7302f 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/ComponentSearchInput.js
+++ b/packages/react-devtools-shared/src/devtools/views/Components/ComponentSearchInput.js
@@ -19,7 +19,8 @@ export default function ComponentSearchInput(props: Props): React.Node {
   const {searchIndex, searchResults, searchText} = useContext(TreeStateContext);
   const dispatch = useContext(TreeDispatcherContext);
 
-  const search = text => dispatch({type: 'SET_SEARCH_TEXT', payload: text});
+  const search = (text: string) =>
+    dispatch({type: 'SET_SEARCH_TEXT', payload: text});
   const goToNextResult = () => dispatch({type: 'GO_TO_NEXT_SEARCH_RESULT'});
   const goToPreviousResult = () =>
     dispatch({type: 'GO_TO_PREVIOUS_SEARCH_RESULT'});
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/Components.js b/packages/react-devtools-shared/src/devtools/views/Components/Components.js
index 9535e598174d3..ba2e99cf34393 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/Components.js
+++ b/packages/react-devtools-shared/src/devtools/views/Components/Components.js
@@ -100,6 +100,7 @@ function Components(_: {}) {
     onResizeEnd = () =>
       dispatch({type: 'ACTION_SET_IS_RESIZING', payload: false});
 
+    // $FlowFixMe[missing-local-annot]
     onResize = event => {
       const resizeElement = resizeElementRef.current;
       const wrapperElement = wrapperElementRef.current;
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/EditableValue.js b/packages/react-devtools-shared/src/devtools/views/Components/EditableValue.js
index 03fe9d7bf7fb2..0511a388a574a 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/EditableValue.js
+++ b/packages/react-devtools-shared/src/devtools/views/Components/EditableValue.js
@@ -36,6 +36,7 @@ export default function EditableValue({
       externalValue: value,
     });
 
+  // $FlowFixMe[missing-local-annot]
   const handleChange = ({target}) =>
     dispatch({
       type: 'UPDATE',
@@ -43,6 +44,7 @@ export default function EditableValue({
       externalValue: value,
     });
 
+  // $FlowFixMe[missing-local-annot]
   const handleCheckBoxToggle = ({target}) => {
     dispatch({
       type: 'UPDATE',
@@ -58,6 +60,7 @@ export default function EditableValue({
     overrideValue(path, target.checked);
   };
 
+  // $FlowFixMe[missing-local-annot]
   const handleKeyDown = event => {
     // Prevent keydown events from e.g. change selected element in the tree
     event.stopPropagation();
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/Element.js b/packages/react-devtools-shared/src/devtools/views/Components/Element.js
index bb850398df32e..c43164b511039 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/Element.js
+++ b/packages/react-devtools-shared/src/devtools/views/Components/Element.js
@@ -75,6 +75,7 @@ export default function Element({data, index, style}: Props): React.Node {
     }
   };
 
+  // $FlowFixMe[missing-local-annot]
   const handleClick = ({metaKey}) => {
     if (id !== null) {
       logEvent({
@@ -99,6 +100,7 @@ export default function Element({data, index, style}: Props): React.Node {
     setIsHovered(false);
   };
 
+  // $FlowFixMe[missing-local-annot]
   const handleKeyDoubleClick = event => {
     // Double clicks on key value are used for text selection (if the text has been truncated).
     // They should not enter the owners tree view.
@@ -220,6 +222,7 @@ export default function Element({data, index, style}: Props): React.Node {
 }
 
 // Prevent double clicks on toggle from drilling into the owner list.
+// $FlowFixMe[missing-local-annot]
 const swallowDoubleClick = event => {
   event.preventDefault();
   event.stopPropagation();
@@ -233,6 +236,7 @@ type ExpandCollapseToggleProps = {
 function ExpandCollapseToggle({element, store}: ExpandCollapseToggleProps) {
   const {children, id, isCollapsed} = element;
 
+  // $FlowFixMe[missing-local-annot]
   const toggleCollapsed = event => {
     event.preventDefault();
     event.stopPropagation();
@@ -240,6 +244,7 @@ function ExpandCollapseToggle({element, store}: ExpandCollapseToggleProps) {
     store.toggleIsCollapsed(id, !isCollapsed);
   };
 
+  // $FlowFixMe[missing-local-annot]
   const stopPropagation = event => {
     // Prevent the row from selecting
     event.stopPropagation();
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/InspectedElementContextTree.js b/packages/react-devtools-shared/src/devtools/views/Components/InspectedElementContextTree.js
index f04a0fd8e067b..eb74c244d9d1a 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/InspectedElementContextTree.js
+++ b/packages/react-devtools-shared/src/devtools/views/Components/InspectedElementContextTree.js
@@ -53,6 +53,7 @@ export default function InspectedElementContextTree({
   // We add an object with a "value" key as a wrapper around Context data
   // so that we can use the shared <KeyValue> component to display it.
   // This wrapper object can't be renamed.
+  // $FlowFixMe[missing-local-annot]
   const canRenamePathsAtDepth = depth => depth > 1;
 
   if (isEmpty) {
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/InspectedElementHooksTree.js b/packages/react-devtools-shared/src/devtools/views/Components/InspectedElementHooksTree.js
index f97a27c372a4c..6744cf4bc6477 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/InspectedElementHooksTree.js
+++ b/packages/react-devtools-shared/src/devtools/views/Components/InspectedElementHooksTree.js
@@ -223,6 +223,7 @@ function HookView({
 
   // Certain hooks are not editable at all (as identified by react-debug-tools).
   // Primitive hook names (e.g. the "State" name for useState) are also never editable.
+  // $FlowFixMe[missing-local-annot]
   const canRenamePathsAtDepth = depth => isStateEditable && depth > 1;
 
   const isCustomHook = subHooks.length > 0;
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/InspectedElementSuspenseToggle.js b/packages/react-devtools-shared/src/devtools/views/Components/InspectedElementSuspenseToggle.js
index b400c6c7f3294..09808d6e3dfdd 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/InspectedElementSuspenseToggle.js
+++ b/packages/react-devtools-shared/src/devtools/views/Components/InspectedElementSuspenseToggle.js
@@ -39,7 +39,7 @@ export default function InspectedElementSuspenseToggle({
 
   const isSuspended = state !== null;
 
-  const toggleSuspense = (path, value) => {
+  const toggleSuspense = (path: any, value: boolean) => {
     const rendererID = store.getRendererIDForElement(id);
     if (rendererID !== null) {
       bridge.send('overrideSuspense', {
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/KeyValue.js b/packages/react-devtools-shared/src/devtools/views/Components/KeyValue.js
index 26c65c15ff96f..ca2f623c1a4c1 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/KeyValue.js
+++ b/packages/react-devtools-shared/src/devtools/views/Components/KeyValue.js
@@ -138,7 +138,7 @@ export default function KeyValue({
     paddingLeft: `${(depth - 1) * 0.75}rem`,
   };
 
-  const overrideValue = (newPath, newValue) => {
+  const overrideValue = (newPath: Array<string | number>, newValue: any) => {
     if (hookID != null) {
       newPath = parseHookPathForEdit(newPath);
     }
@@ -156,7 +156,7 @@ export default function KeyValue({
     }
   };
 
-  const deletePath = pathToDelete => {
+  const deletePath = (pathToDelete: Array<string | number>) => {
     if (hookID != null) {
       pathToDelete = parseHookPathForEdit(pathToDelete);
     }
@@ -173,7 +173,10 @@ export default function KeyValue({
     }
   };
 
-  const renamePath = (oldPath, newPath) => {
+  const renamePath = (
+    oldPath: Array<string | number>,
+    newPath: Array<string | number>,
+  ) => {
     if (newPath[newPath.length - 1] === '') {
       // Deleting the key suggests an intent to delete the whole path.
       if (canDeletePaths) {
@@ -477,7 +480,9 @@ export default function KeyValue({
   return children;
 }
 
+// $FlowFixMe[missing-local-annot]
 function DeleteToggle({deletePath, name, path}) {
+  // $FlowFixMe[missing-local-annot]
   const handleClick = event => {
     event.stopPropagation();
     deletePath(path);
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/NativeStyleEditor/AutoSizeInput.js b/packages/react-devtools-shared/src/devtools/views/Components/NativeStyleEditor/AutoSizeInput.js
index 057dd4625f2c3..059cb42273f44 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/NativeStyleEditor/AutoSizeInput.js
+++ b/packages/react-devtools-shared/src/devtools/views/Components/NativeStyleEditor/AutoSizeInput.js
@@ -27,6 +27,7 @@ export default function AutoSizeInput({
   value,
   ...rest
 }: Props): React.Node {
+  // $FlowFixMe[missing-local-annot]
   const onFocusWrapper = event => {
     const input = event.target;
     if (input !== null) {
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/NativeStyleEditor/StyleEditor.js b/packages/react-devtools-shared/src/devtools/views/Components/NativeStyleEditor/StyleEditor.js
index 7b85633f214a8..5b7cdf8563412 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/NativeStyleEditor/StyleEditor.js
+++ b/packages/react-devtools-shared/src/devtools/views/Components/NativeStyleEditor/StyleEditor.js
@@ -171,6 +171,7 @@ function Row({
   const [isAttributeValid, setIsAttributeValid] = useState(true);
   const [isValueValid, setIsValueValid] = useState(true);
 
+  // $FlowFixMe[missing-local-annot]
   const validateAndSetLocalAttribute = newAttribute => {
     const isValid =
       newAttribute === '' ||
@@ -183,6 +184,7 @@ function Row({
     });
   };
 
+  // $FlowFixMe[missing-local-annot]
   const validateAndSetLocalValue = newValue => {
     let isValid = false;
     try {
@@ -262,6 +264,7 @@ function Field({
   placeholder,
   value,
 }: FieldProps) {
+  // $FlowFixMe[missing-local-annot]
   const onKeyDown = event => {
     switch (event.key) {
       case 'Enter':
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/NewArrayValue.js b/packages/react-devtools-shared/src/devtools/views/Components/NewArrayValue.js
index 0a6cdf842fa01..c8247f7449623 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/NewArrayValue.js
+++ b/packages/react-devtools-shared/src/devtools/views/Components/NewArrayValue.js
@@ -46,7 +46,8 @@ export default function NewArrayValue({
 
   // This is a bit of an unusual usage of the EditableName component,
   // but otherwise it acts the way we want for a new Array entry.
-  const overrideName = (oldPath, newPath) => {
+  // $FlowFixMe[missing-local-annot]
+  const overrideName = (oldPath: any, newPath) => {
     const value = newPath[newPath.length - 1];
 
     let parsedValue;
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/NewKeyValue.js b/packages/react-devtools-shared/src/devtools/views/Components/NewKeyValue.js
index 6435e6da4bf59..8fb4cc1dc8ca9 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/NewKeyValue.js
+++ b/packages/react-devtools-shared/src/devtools/views/Components/NewKeyValue.js
@@ -42,11 +42,15 @@ export default function NewKeyValue({
   const [newPropKey, setNewPropKey] = useState<number>(0);
   const [newPropName, setNewPropName] = useState<string>('');
 
-  const overrideNewEntryName = (oldPath, newPath) => {
+  // $FlowFixMe[missing-local-annot]
+  const overrideNewEntryName = (oldPath: any, newPath) => {
     setNewPropName(newPath[newPath.length - 1]);
   };
 
-  const overrideNewEntryValue = (newPath, value) => {
+  const overrideNewEntryValue = (
+    newPath: Array<string | number>,
+    value: any,
+  ) => {
     if (!newPropName) {
       return;
     }
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/OwnersStack.js b/packages/react-devtools-shared/src/devtools/views/Components/OwnersStack.js
index f79a307885c91..c557e374ab2f5 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/OwnersStack.js
+++ b/packages/react-devtools-shared/src/devtools/views/Components/OwnersStack.js
@@ -56,7 +56,7 @@ type State = {
   selectedIndex: number,
 };
 
-function dialogReducer(state, action) {
+function dialogReducer(state: State, action: Action) {
   switch (action.type) {
     case 'UPDATE_OWNER_ID':
       const selectedIndex = action.owners.findIndex(
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/Tree.js b/packages/react-devtools-shared/src/devtools/views/Components/Tree.js
index 106bda6d36123..7374e4d70ff32 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/Tree.js
+++ b/packages/react-devtools-shared/src/devtools/views/Components/Tree.js
@@ -101,7 +101,7 @@ export default function Tree(props: Props): React.Node {
   // Picking an element in the inspector should put focus into the tree.
   // This ensures that keyboard navigation works right after picking a node.
   useEffect(() => {
-    function handleStopInspectingNative(didSelectNode) {
+    function handleStopInspectingNative(didSelectNode: boolean) {
       if (didSelectNode && focusTargetRef.current !== null) {
         focusTargetRef.current.focus();
         logEvent({
@@ -540,6 +540,7 @@ function updateIndentationSizeVar(
   list.style.setProperty('--indentation-size', `${maxIndentationSize}px`);
 }
 
+// $FlowFixMe[missing-local-annot]
 function InnerElementType({children, style, ...rest}) {
   const {ownerID} = useContext(TreeStateContext);
 
diff --git a/packages/react-devtools-shared/src/devtools/views/ModalDialog.js b/packages/react-devtools-shared/src/devtools/views/ModalDialog.js
index c7d3f7befb7a2..cf927e055de71 100644
--- a/packages/react-devtools-shared/src/devtools/views/ModalDialog.js
+++ b/packages/react-devtools-shared/src/devtools/views/ModalDialog.js
@@ -61,7 +61,7 @@ const ModalDialogContext: ReactContext<ModalDialogContextType> = createContext<M
 );
 ModalDialogContext.displayName = 'ModalDialogContext';
 
-function dialogReducer(state, action) {
+function dialogReducer(state: State, action: Action) {
   switch (action.type) {
     case 'HIDE':
       return {
diff --git a/packages/react-devtools-shared/src/devtools/views/Profiler/CommitTreeBuilder.js b/packages/react-devtools-shared/src/devtools/views/Profiler/CommitTreeBuilder.js
index b7806a98222ed..5d2e6e9cf5bb1 100644
--- a/packages/react-devtools-shared/src/devtools/views/Profiler/CommitTreeBuilder.js
+++ b/packages/react-devtools-shared/src/devtools/views/Profiler/CommitTreeBuilder.js
@@ -28,7 +28,7 @@ import type {
   ProfilingDataForRootFrontend,
 } from 'react-devtools-shared/src/devtools/views/Profiler/types';
 
-const debug = (methodName, ...args) => {
+const debug = (methodName: string, ...args: Array<string>) => {
   if (__DEBUG__) {
     console.log(
       `%cCommitTreeBuilder %c${methodName}`,
diff --git a/packages/react-devtools-shared/src/devtools/views/Profiler/SidebarEventInfo.js b/packages/react-devtools-shared/src/devtools/views/Profiler/SidebarEventInfo.js
index 716bf67adee76..d0d8be666ed32 100644
--- a/packages/react-devtools-shared/src/devtools/views/Profiler/SidebarEventInfo.js
+++ b/packages/react-devtools-shared/src/devtools/views/Profiler/SidebarEventInfo.js
@@ -7,6 +7,7 @@
  * @flow
  */
 
+import type {Stack} from '../../utils';
 import type {SchedulingEvent} from 'react-devtools-timeline/src/types';
 
 import * as React from 'react';
@@ -35,7 +36,7 @@ function SchedulingEventInfo({eventInfo}: SchedulingEventProps) {
   const {componentName, timestamp} = eventInfo;
   const componentStack = eventInfo.componentStack || null;
 
-  const viewSource = source => {
+  const viewSource = (source: ?Stack) => {
     if (viewUrlSourceFunction != null && source != null) {
       viewUrlSourceFunction(...source);
     }
diff --git a/packages/react-devtools-shared/src/devtools/views/Profiler/SidebarSelectedFiberInfo.js b/packages/react-devtools-shared/src/devtools/views/Profiler/SidebarSelectedFiberInfo.js
index bfdb4a6e31684..930183ee0cc65 100644
--- a/packages/react-devtools-shared/src/devtools/views/Profiler/SidebarSelectedFiberInfo.js
+++ b/packages/react-devtools-shared/src/devtools/views/Profiler/SidebarSelectedFiberInfo.js
@@ -38,6 +38,7 @@ export default function SidebarSelectedFiberInfo(_: Props): React.Node {
     rootID: ((rootID: any): number),
   });
 
+  // $FlowFixMe[missing-local-annot]
   const handleKeyDown = event => {
     switch (event.key) {
       case 'ArrowUp':
diff --git a/packages/react-devtools-shared/src/devtools/views/Profiler/SnapshotSelector.js b/packages/react-devtools-shared/src/devtools/views/Profiler/SnapshotSelector.js
index 052c0c100c648..57b26ce15fa3d 100644
--- a/packages/react-devtools-shared/src/devtools/views/Profiler/SnapshotSelector.js
+++ b/packages/react-devtools-shared/src/devtools/views/Profiler/SnapshotSelector.js
@@ -86,6 +86,7 @@ export default function SnapshotSelector(_: Props): React.Node {
 
   let label = null;
   if (numFilteredCommits > 0) {
+    // $FlowFixMe[missing-local-annot]
     const handleCommitInputChange = event => {
       const value = parseInt(event.currentTarget.value, 10);
       if (!isNaN(value)) {
@@ -100,10 +101,12 @@ export default function SnapshotSelector(_: Props): React.Node {
       }
     };
 
+    // $FlowFixMe[missing-local-annot]
     const handleClick = event => {
       event.currentTarget.select();
     };
 
+    // $FlowFixMe[missing-local-annot]
     const handleKeyDown = event => {
       switch (event.key) {
         case 'ArrowDown':
@@ -159,6 +162,7 @@ export default function SnapshotSelector(_: Props): React.Node {
     selectCommitIndex(filteredCommitIndices[nextCommitIndex]);
   };
 
+  // $FlowFixMe[missing-local-annot]
   const handleKeyDown = event => {
     switch (event.key) {
       case 'ArrowLeft':
diff --git a/packages/react-devtools-shared/src/devtools/views/Profiler/Tooltip.js b/packages/react-devtools-shared/src/devtools/views/Profiler/Tooltip.js
index baf61f82822be..b43f51cb04e23 100644
--- a/packages/react-devtools-shared/src/devtools/views/Profiler/Tooltip.js
+++ b/packages/react-devtools-shared/src/devtools/views/Profiler/Tooltip.js
@@ -55,7 +55,15 @@ export default function Tooltip({
 const TOOLTIP_OFFSET = 5;
 
 // Method used to find the position of the tooltip based on current mouse position
-function getTooltipPosition(element, mousePosition) {
+function getTooltipPosition(
+  element: empty,
+  mousePosition: {
+    height: number,
+    mouseX: number,
+    mouseY: number,
+    width: number,
+  },
+) {
   const {height, mouseX, mouseY, width} = mousePosition;
   let top: number | string = 0;
   let left: number | string = 0;
@@ -85,7 +93,7 @@ function getTooltipPosition(element, mousePosition) {
 
 // method used to find the current mouse position inside the container
 function getMousePosition(
-  relativeContainer,
+  relativeContainer: null,
   mouseEvent: SyntheticMouseEvent<EventTarget>,
 ) {
   if (relativeContainer !== null) {
diff --git a/packages/react-devtools-shared/src/devtools/views/SearchInput.js b/packages/react-devtools-shared/src/devtools/views/SearchInput.js
index 2569b685536ec..8a43202f87c30 100644
--- a/packages/react-devtools-shared/src/devtools/views/SearchInput.js
+++ b/packages/react-devtools-shared/src/devtools/views/SearchInput.js
@@ -40,9 +40,11 @@ export default function SearchInput({
 
   const resetSearch = () => search('');
 
+  // $FlowFixMe[missing-local-annot]
   const handleChange = ({currentTarget}) => {
     search(currentTarget.value);
   };
+  // $FlowFixMe[missing-local-annot]
   const handleKeyPress = ({key, shiftKey}) => {
     if (key === 'Enter') {
       if (shiftKey) {
diff --git a/packages/react-devtools-shared/src/devtools/views/hooks.js b/packages/react-devtools-shared/src/devtools/views/hooks.js
index 6cc82dd424dd8..c5ee3a3e83d7a 100644
--- a/packages/react-devtools-shared/src/devtools/views/hooks.js
+++ b/packages/react-devtools-shared/src/devtools/views/hooks.js
@@ -43,7 +43,10 @@ type UseEditableValueState = {
   parsedValue: any,
 };
 
-function useEditableValueReducer(state, action) {
+function useEditableValueReducer(
+  state: UseEditableValueState,
+  action: UseEditableValueAction,
+) {
   switch (action.type) {
     case 'RESET':
       return {
@@ -188,6 +191,7 @@ export function useLocalStorage<T>(
   // Listen for changes to this local storage value made from other windows.
   // This enables the e.g. "⚛️ Elements" tab to update in response to changes from "⚛️ Settings".
   useLayoutEffect(() => {
+    // $FlowFixMe[missing-local-annot]
     const onStorage = event => {
       const newValue = getValueFromLocalStorage();
       if (key === event.key && storedValue !== newValue) {
diff --git a/packages/react-devtools-shared/src/hook.js b/packages/react-devtools-shared/src/hook.js
index 547f6b35e63ab..acecd13f06a69 100644
--- a/packages/react-devtools-shared/src/hook.js
+++ b/packages/react-devtools-shared/src/hook.js
@@ -9,7 +9,12 @@
  */
 
 import type {BrowserTheme} from 'react-devtools-shared/src/devtools/views/DevTools';
-import type {DevToolsHook} from 'react-devtools-shared/src/backend/types';
+import type {
+  RendererID,
+  ReactRenderer,
+  Handler,
+  DevToolsHook,
+} from 'react-devtools-shared/src/backend/types';
 
 import {
   patchConsoleUsingWindowValues,
@@ -40,7 +45,7 @@ export function installHook(target: any): DevToolsHook | null {
     }
   }
 
-  function detectReactBuildType(renderer) {
+  function detectReactBuildType(renderer: ReactRenderer) {
     try {
       if (typeof renderer.version === 'string') {
         // React DOM Fiber (16+)
@@ -263,7 +268,7 @@ export function installHook(target: any): DevToolsHook | null {
           ? targetConsole[method].__REACT_DEVTOOLS_STRICT_MODE_ORIGINAL_METHOD__
           : targetConsole[method]);
 
-        const overrideMethod = (...args) => {
+        const overrideMethod = (...args: $ReadOnlyArray<any>) => {
           if (!hideConsoleLogsInStrictMode) {
             // Dim the text color of the double logs if we're not
             // hiding them.
@@ -316,7 +321,7 @@ export function installHook(target: any): DevToolsHook | null {
 
   let uidCounter = 0;
 
-  function inject(renderer) {
+  function inject(renderer: ReactRenderer) {
     const id = ++uidCounter;
     renderers.set(id, renderer);
 
@@ -374,19 +379,19 @@ export function installHook(target: any): DevToolsHook | null {
 
   let hasDetectedBadDCE = false;
 
-  function sub(event, fn) {
+  function sub(event: string, fn: Handler) {
     hook.on(event, fn);
     return () => hook.off(event, fn);
   }
 
-  function on(event, fn) {
+  function on(event: string, fn: Handler) {
     if (!listeners[event]) {
       listeners[event] = [];
     }
     listeners[event].push(fn);
   }
 
-  function off(event, fn) {
+  function off(event: string, fn: Handler) {
     if (!listeners[event]) {
       return;
     }
@@ -399,13 +404,13 @@ export function installHook(target: any): DevToolsHook | null {
     }
   }
 
-  function emit(event, data) {
+  function emit(event: string, data: any) {
     if (listeners[event]) {
       listeners[event].map(fn => fn(data));
     }
   }
 
-  function getFiberRoots(rendererID) {
+  function getFiberRoots(rendererID: RendererID) {
     const roots = fiberRoots;
     if (!roots[rendererID]) {
       roots[rendererID] = new Set();
@@ -413,14 +418,18 @@ export function installHook(target: any): DevToolsHook | null {
     return roots[rendererID];
   }
 
-  function onCommitFiberUnmount(rendererID, fiber) {
+  function onCommitFiberUnmount(rendererID: RendererID, fiber: any) {
     const rendererInterface = rendererInterfaces.get(rendererID);
     if (rendererInterface != null) {
       rendererInterface.handleCommitFiberUnmount(fiber);
     }
   }
 
-  function onCommitFiberRoot(rendererID, root, priorityLevel) {
+  function onCommitFiberRoot(
+    rendererID: RendererID,
+    root: any,
+    priorityLevel: void | number,
+  ) {
     const mountedRoots = hook.getFiberRoots(rendererID);
     const current = root.current;
     const isKnownRoot = mountedRoots.has(root);
@@ -439,14 +448,14 @@ export function installHook(target: any): DevToolsHook | null {
     }
   }
 
-  function onPostCommitFiberRoot(rendererID, root) {
+  function onPostCommitFiberRoot(rendererID: RendererID, root: any) {
     const rendererInterface = rendererInterfaces.get(rendererID);
     if (rendererInterface != null) {
       rendererInterface.handlePostCommitFiberRoot(root);
     }
   }
 
-  function setStrictMode(rendererID, isStrictMode) {
+  function setStrictMode(rendererID: RendererID, isStrictMode: any) {
     const rendererInterface = rendererInterfaces.get(rendererID);
     if (rendererInterface != null) {
       if (isStrictMode) {
diff --git a/packages/react-devtools-shared/src/node_modules/react-window/src/createGridComponent.js b/packages/react-devtools-shared/src/node_modules/react-window/src/createGridComponent.js
index d1da857a5496a..a4c90eba85f32 100644
--- a/packages/react-devtools-shared/src/node_modules/react-window/src/createGridComponent.js
+++ b/packages/react-devtools-shared/src/node_modules/react-window/src/createGridComponent.js
@@ -143,7 +143,13 @@ type ValidateProps = (props: Props<any>) => void;
 
 const IS_SCROLLING_DEBOUNCE_INTERVAL = 150;
 
-const defaultItemKey = ({ columnIndex, data, rowIndex }) =>
+const defaultItemKey = (
+  {
+    columnIndex,
+    data,
+    rowIndex
+  }: { columnIndex: number, data: any, rowIndex: number },
+) =>
   `${rowIndex}:${columnIndex}`;
 
 // In DEV mode, this Set helps us only log a warning once per component instance.
@@ -197,7 +203,7 @@ export default function createGridComponent({
     _resetIsScrollingTimeoutId: TimeoutID | null = null;
     _outerRef: ?HTMLDivElement;
 
-    static defaultProps = {
+    static defaultProps: { direction: string, itemData: void, useIsScrolling: boolean } = {
       direction: 'ltr',
       itemData: undefined,
       useIsScrolling: false,
@@ -393,7 +399,7 @@ export default function createGridComponent({
       }
     }
 
-    render() {
+    render(): any {
       const {
         children,
         className,
@@ -486,7 +492,7 @@ export default function createGridComponent({
       );
     }
 
-    _callOnItemsRendered: (
+    _callOnItemsRendered: ((
       overscanColumnStartIndex: number,
       overscanColumnStopIndex: number,
       overscanRowStartIndex: number,
@@ -495,8 +501,7 @@ export default function createGridComponent({
       visibleColumnStopIndex: number,
       visibleRowStartIndex: number,
       visibleRowStopIndex: number
-    ) => void;
-    _callOnItemsRendered = memoizeOne(
+    ) => void) = memoizeOne(
       (
         overscanColumnStartIndex: number,
         overscanColumnStopIndex: number,
@@ -519,14 +524,13 @@ export default function createGridComponent({
         })
     );
 
-    _callOnScroll: (
+    _callOnScroll: ((
       scrollLeft: number,
       scrollTop: number,
       horizontalScrollDirection: ScrollDirection,
       verticalScrollDirection: ScrollDirection,
       scrollUpdateWasRequested: boolean
-    ) => void;
-    _callOnScroll = memoizeOne(
+    ) => void) = memoizeOne(
       (
         scrollLeft: number,
         scrollTop: number,
@@ -595,7 +599,6 @@ export default function createGridComponent({
     // So that pure component sCU will prevent re-renders.
     // We maintain this cache, and pass a style prop rather than index,
     // So that List can clear cached styles and force item re-render if necessary.
-    _getItemStyle: (rowIndex: number, columnIndex: number) => Object;
     _getItemStyle = (rowIndex: number, columnIndex: number): Object => {
       const { columnWidth, direction, rowHeight } = this.props;
 
@@ -628,8 +631,8 @@ export default function createGridComponent({
       return style;
     };
 
-    _getItemStyleCache: (_: any, __: any, ___: any) => ItemStyleCache;
-    _getItemStyleCache = memoizeOne((_: any, __: any, ___: any) => ({}));
+    _getItemStyleCache: ((_: any, __: any, ___: any) => ItemStyleCache) =
+      memoizeOne((_: any, __: any, ___: any) => ({}));
 
     _getHorizontalRangeToRender(): [number, number, number, number] {
       const {
diff --git a/packages/react-devtools-shared/src/node_modules/react-window/src/createListComponent.js b/packages/react-devtools-shared/src/node_modules/react-window/src/createListComponent.js
index 11ff59317c5f5..210dba2b642bc 100644
--- a/packages/react-devtools-shared/src/node_modules/react-window/src/createListComponent.js
+++ b/packages/react-devtools-shared/src/node_modules/react-window/src/createListComponent.js
@@ -162,7 +162,13 @@ export default function createListComponent({
     _outerRef: ?HTMLDivElement;
     _resetIsScrollingTimeoutId: TimeoutID | null = null;
 
-    static defaultProps = {
+    static defaultProps: {
+      direction: string,
+      itemData: void,
+      layout: string,
+      overscanCount: number,
+      useIsScrolling: boolean,
+    } = {
       direction: 'ltr',
       itemData: undefined,
       layout: 'vertical',
@@ -288,7 +294,7 @@ export default function createListComponent({
       }
     }
 
-    render() {
+    render(): any {
       const {
         children,
         className,
@@ -370,13 +376,12 @@ export default function createListComponent({
       );
     }
 
-    _callOnItemsRendered: (
+    _callOnItemsRendered: ((
       overscanStartIndex: number,
       overscanStopIndex: number,
       visibleStartIndex: number,
       visibleStopIndex: number
-    ) => void;
-    _callOnItemsRendered = memoizeOne(
+    ) => void) = memoizeOne(
       (
         overscanStartIndex: number,
         overscanStopIndex: number,
@@ -391,12 +396,11 @@ export default function createListComponent({
         })
     );
 
-    _callOnScroll: (
+    _callOnScroll: ((
       scrollDirection: ScrollDirection,
       scrollOffset: number,
       scrollUpdateWasRequested: boolean
-    ) => void;
-    _callOnScroll = memoizeOne(
+    ) => void) = memoizeOne(
       (
         scrollDirection: ScrollDirection,
         scrollOffset: number,
@@ -446,7 +450,6 @@ export default function createListComponent({
     // So that pure component sCU will prevent re-renders.
     // We maintain this cache, and pass a style prop rather than index,
     // So that List can clear cached styles and force item re-render if necessary.
-    _getItemStyle: (index: number) => Object;
     _getItemStyle = (index: number): Object => {
       const { direction, itemSize, layout } = this.props;
 
@@ -480,8 +483,8 @@ export default function createListComponent({
       return style;
     };
 
-    _getItemStyleCache: (_: any, __: any, ___: any) => ItemStyleCache;
-    _getItemStyleCache = memoizeOne((_: any, __: any, ___: any) => ({}));
+    _getItemStyleCache: ((_: any, __: any, ___: any) => ItemStyleCache) =
+      memoizeOne((_: any, __: any, ___: any) => ({}));
 
     _getRangeToRender(): [number, number, number, number] {
       const { itemCount, overscanCount } = this.props;
diff --git a/packages/react-devtools-shared/src/node_modules/react-window/src/shouldComponentUpdate.js b/packages/react-devtools-shared/src/node_modules/react-window/src/shouldComponentUpdate.js
index 6b891bd4d2e50..01ca6d5071268 100644
--- a/packages/react-devtools-shared/src/node_modules/react-window/src/shouldComponentUpdate.js
+++ b/packages/react-devtools-shared/src/node_modules/react-window/src/shouldComponentUpdate.js
@@ -7,8 +7,9 @@ import shallowDiffers from './shallowDiffers';
 // It knows to compare individual style props and ignore the wrapper object.
 // See https://reactjs.org/docs/react-component.html#shouldcomponentupdate
 export default function shouldComponentUpdate(
+  this: interface { props: any, state: any },
   nextProps: Object,
-  nextState: Object
+  nextState: Object,
 ): boolean {
   return (
     !areEqual(this.props, nextProps) || shallowDiffers(this.state, nextState)
diff --git a/packages/react-devtools-shared/src/registerDevToolsEventLogger.js b/packages/react-devtools-shared/src/registerDevToolsEventLogger.js
index 8f261c1ed90d5..6bc08d91524b7 100644
--- a/packages/react-devtools-shared/src/registerDevToolsEventLogger.js
+++ b/packages/react-devtools-shared/src/registerDevToolsEventLogger.js
@@ -56,7 +56,7 @@ export function registerDevToolsEventLogger(
     }
   }
 
-  function handleLoggingIFrameLoaded(iframe) {
+  function handleLoggingIFrameLoaded(iframe: HTMLIFrameElement) {
     if (loggingIFrame != null) {
       return;
     }
diff --git a/packages/react-devtools-shell/src/app/DeeplyNestedComponents/index.js b/packages/react-devtools-shell/src/app/DeeplyNestedComponents/index.js
index dc1de76dfa733..8c9a02e5e0c79 100644
--- a/packages/react-devtools-shell/src/app/DeeplyNestedComponents/index.js
+++ b/packages/react-devtools-shell/src/app/DeeplyNestedComponents/index.js
@@ -10,19 +10,19 @@
 import * as React from 'react';
 import {Fragment} from 'react';
 
-function wrapWithHoc(Component, index) {
+function wrapWithHoc(Component: () => any, index: number) {
   function HOC() {
     return <Component />;
   }
 
-  const displayName = Component.displayName || Component.name;
+  const displayName = (Component: any).displayName || Component.name;
 
   // $FlowFixMe[incompatible-type] found when upgrading Flow
   HOC.displayName = `withHoc${index}(${displayName})`;
   return HOC;
 }
 
-function wrapWithNested(Component, times) {
+function wrapWithNested(Component: () => any, times: number) {
   for (let i = 0; i < times; i++) {
     Component = wrapWithHoc(Component, i);
   }
diff --git a/packages/react-devtools-shell/src/app/EditableProps/index.js b/packages/react-devtools-shell/src/app/EditableProps/index.js
index fe617337faeb2..5cfad3f7e837f 100644
--- a/packages/react-devtools-shell/src/app/EditableProps/index.js
+++ b/packages/react-devtools-shell/src/app/EditableProps/index.js
@@ -7,6 +7,7 @@
  * @flow
  */
 
+import type {ReactContext} from 'shared/ReactTypes';
 import * as React from 'react';
 import {
   createContext,
@@ -23,7 +24,8 @@ import {
 
 const initialData = {foo: 'FOO', bar: 'BAR'};
 
-function reducer(state, action) {
+// $FlowFixMe[missing-local-annot]
+function reducer(state, action: {type: string}) {
   switch (action.type) {
     case 'swap':
       return {foo: state.bar, bar: state.foo};
@@ -72,19 +74,20 @@ type Props = {name: string, toggle: boolean};
 type State = {cities: Array<string>, state: string};
 
 class StatefulClass extends Component<Props, State> {
-  static contextType = BoolContext;
+  static contextType: ReactContext<boolean> = BoolContext;
 
   state: State = {
     cities: ['San Francisco', 'San Jose'],
     state: 'California',
   };
 
-  handleChange = ({target}) =>
+  // $FlowFixMe[missing-local-annot]
+  handleChange = ({target}): any =>
     this.setState({
       state: target.value,
     });
 
-  render() {
+  render(): any {
     return (
       <ul>
         <li>Name: {this.props.name}</li>
@@ -141,7 +144,7 @@ export default function EditableProps(): React.Node {
 }
 
 // Below copied from https://usehooks.com/
-function useDebounce(value, delay) {
+function useDebounce(value: number, delay: number) {
   // State and setters for debounced value
   const [debouncedValue, setDebouncedValue] = useState(value);
 
diff --git a/packages/react-devtools-shell/src/app/ElementTypes/index.js b/packages/react-devtools-shell/src/app/ElementTypes/index.js
index 6268c75e88b66..3a5edef08e603 100644
--- a/packages/react-devtools-shell/src/app/ElementTypes/index.js
+++ b/packages/react-devtools-shell/src/app/ElementTypes/index.js
@@ -25,7 +25,7 @@ const Context = createContext('abc');
 Context.displayName = 'ExampleContext';
 
 class ClassComponent extends Component<any> {
-  render() {
+  render(): null {
     return null;
   }
 }
diff --git a/packages/react-devtools-shell/src/app/ErrorBoundaries/index.js b/packages/react-devtools-shell/src/app/ErrorBoundaries/index.js
index bdd9666fbec7e..a87d853ea1b25 100644
--- a/packages/react-devtools-shell/src/app/ErrorBoundaries/index.js
+++ b/packages/react-devtools-shell/src/app/ErrorBoundaries/index.js
@@ -11,13 +11,13 @@ import * as React from 'react';
 import {Fragment} from 'react';
 
 class ErrorBoundary extends React.Component {
-  state = {hasError: false};
+  state: {hasError: boolean} = {hasError: false};
 
-  static getDerivedStateFromError(error) {
+  static getDerivedStateFromError(error: any): {hasError: boolean} {
     return {hasError: true};
   }
 
-  render() {
+  render(): any {
     const {hasError} = this.state;
     if (hasError) {
       return (
@@ -49,6 +49,7 @@ class ErrorBoundary extends React.Component {
   }
 }
 
+// $FlowFixMe[missing-local-annot]
 function Component({label}) {
   return <div>{label}</div>;
 }
diff --git a/packages/react-devtools-shell/src/app/Iframe/index.js b/packages/react-devtools-shell/src/app/Iframe/index.js
index 50beadd137aa2..99eafee6b29b7 100644
--- a/packages/react-devtools-shell/src/app/Iframe/index.js
+++ b/packages/react-devtools-shell/src/app/Iframe/index.js
@@ -19,6 +19,7 @@ export default function Iframe(): React.Node {
 
 const iframeStyle = {border: '2px solid #eee', height: 80};
 
+// $FlowFixMe[missing-local-annot]
 function Frame(props) {
   const [element, setElement] = React.useState(null);
 
diff --git a/packages/react-devtools-shell/src/app/InlineWarnings/index.js b/packages/react-devtools-shell/src/app/InlineWarnings/index.js
index 6b7b189f70174..ddfac6acb4c3b 100644
--- a/packages/react-devtools-shell/src/app/InlineWarnings/index.js
+++ b/packages/react-devtools-shell/src/app/InlineWarnings/index.js
@@ -3,11 +3,13 @@
 import * as React from 'react';
 import {Fragment, useEffect, useRef, useState} from 'react';
 
+// $FlowFixMe[missing-local-annot]
 function WarnDuringRender({children = null}) {
   console.warn('This warning fires during every render');
   return children;
 }
 
+// $FlowFixMe[missing-local-annot]
 function WarnOnMount({children = null}) {
   useEffect(() => {
     console.warn('This warning fires on initial mount only');
@@ -15,6 +17,7 @@ function WarnOnMount({children = null}) {
   return children;
 }
 
+// $FlowFixMe[missing-local-annot]
 function WarnOnUpdate({children = null}) {
   const didMountRef = useRef(false);
   useEffect(() => {
@@ -27,6 +30,7 @@ function WarnOnUpdate({children = null}) {
   return children;
 }
 
+// $FlowFixMe[missing-local-annot]
 function WarnOnUnmount({children = null}) {
   useEffect(() => {
     return () => {
@@ -36,11 +40,13 @@ function WarnOnUnmount({children = null}) {
   return children;
 }
 
+// $FlowFixMe[missing-local-annot]
 function ErrorDuringRender({children = null}) {
   console.error('This error fires during every render');
   return children;
 }
 
+// $FlowFixMe[missing-local-annot]
 function ErrorOnMount({children = null}) {
   useEffect(() => {
     console.error('This error fires on initial mount only');
@@ -48,6 +54,7 @@ function ErrorOnMount({children = null}) {
   return children;
 }
 
+// $FlowFixMe[missing-local-annot]
 function ErrorOnUpdate({children = null}) {
   const didMountRef = useRef(false);
   useEffect(() => {
@@ -60,6 +67,7 @@ function ErrorOnUpdate({children = null}) {
   return children;
 }
 
+// $FlowFixMe[missing-local-annot]
 function ErrorOnUnmount({children = null}) {
   useEffect(() => {
     return () => {
@@ -69,12 +77,14 @@ function ErrorOnUnmount({children = null}) {
   return children;
 }
 
+// $FlowFixMe[missing-local-annot]
 function ErrorAndWarningDuringRender({children = null}) {
   console.warn('This warning fires during every render');
   console.error('This error fires during every render');
   return children;
 }
 
+// $FlowFixMe[missing-local-annot]
 function ErrorAndWarningOnMount({children = null}) {
   useEffect(() => {
     console.warn('This warning fires on initial mount only');
@@ -83,6 +93,7 @@ function ErrorAndWarningOnMount({children = null}) {
   return children;
 }
 
+// $FlowFixMe[missing-local-annot]
 function ErrorAndWarningOnUpdate({children = null}) {
   const didMountRef = useRef(false);
   useEffect(() => {
@@ -96,6 +107,7 @@ function ErrorAndWarningOnUpdate({children = null}) {
   return children;
 }
 
+// $FlowFixMe[missing-local-annot]
 function ErrorAndWarningOnUnmount({children = null}) {
   useEffect(() => {
     return () => {
@@ -106,6 +118,7 @@ function ErrorAndWarningOnUnmount({children = null}) {
   return children;
 }
 
+// $FlowFixMe[missing-local-annot]
 function ReallyLongErrorMessageThatWillCauseTextToBeTruncated({
   children = null,
 }) {
@@ -115,20 +128,24 @@ function ReallyLongErrorMessageThatWillCauseTextToBeTruncated({
   return children;
 }
 
+// $FlowFixMe[missing-local-annot]
 function ErrorWithMultipleArgs({children = null}) {
   console.error('This error', 'passes console', 4, 'arguments');
   return children;
 }
 
+// $FlowFixMe[missing-local-annot]
 function ErrorWithStringSubstitutions({children = null}) {
   console.error('This error uses "%s" substitutions', 'string');
   return children;
 }
 
+// $FlowFixMe[missing-local-annot]
 function ReactErrorOnHostComponent({children = null}) {
   return <div data-camelCasedAttribute="should-lower-case">{children}</div>;
 }
 
+// $FlowFixMe[missing-local-annot]
 function DuplicateWarningsAndErrors({children = null}) {
   console.warn('this warning is logged twice per render');
   console.warn('this warning is logged twice per render');
@@ -137,6 +154,7 @@ function DuplicateWarningsAndErrors({children = null}) {
   return <div data-camelCasedAttribute="should-lower-case">{children}</div>;
 }
 
+// $FlowFixMe[missing-local-annot]
 function MultipleWarningsAndErrors({children = null}) {
   console.warn('this is the first warning logged');
   console.warn('this is the second warning logged');
@@ -145,6 +163,7 @@ function MultipleWarningsAndErrors({children = null}) {
   return <div data-camelCasedAttribute="should-lower-case">{children}</div>;
 }
 
+// $FlowFixMe[missing-local-annot]
 function ComponentWithMissingKey({children}) {
   return [<div />];
 }
diff --git a/packages/react-devtools-shell/src/app/InspectableElements/Contexts.js b/packages/react-devtools-shell/src/app/InspectableElements/Contexts.js
index da54beed0b213..fc76fc11c5476 100644
--- a/packages/react-devtools-shell/src/app/InspectableElements/Contexts.js
+++ b/packages/react-devtools-shell/src/app/InspectableElements/Contexts.js
@@ -11,9 +11,11 @@ import * as React from 'react';
 import {createContext, Component, useContext, useState} from 'react';
 import PropTypes from 'prop-types';
 
+import type {ReactContext} from 'shared/ReactTypes';
+
 function someNamedFunction() {}
 
-function formatContextForDisplay(name, value) {
+function formatContextForDisplay(name: string, value: any | string) {
   return (
     <li>
       {name}: <pre>{JSON.stringify(value, null, 2)}</pre>
@@ -34,7 +36,17 @@ const contextData = {
 };
 
 class LegacyContextProvider extends Component<any> {
-  static childContextTypes = {
+  static childContextTypes: {
+    array: any,
+    bool: any,
+    func: any,
+    null: any,
+    number: any,
+    object: any,
+    string: any,
+    symbol: any,
+    undefined: any,
+  } = {
     array: PropTypes.array,
     bool: PropTypes.bool,
     func: PropTypes.func,
@@ -46,17 +58,37 @@ class LegacyContextProvider extends Component<any> {
     undefined: PropTypes.any,
   };
 
-  getChildContext() {
+  getChildContext(): {
+    array: Array<string>,
+    bool: boolean,
+    func: () => void,
+    null: null,
+    number: number,
+    object: {outer: {inner: {...}}},
+    string: string,
+    symbol: symbol,
+    undefined: void,
+  } {
     return contextData;
   }
 
-  render() {
+  render(): any {
     return this.props.children;
   }
 }
 
 class LegacyContextConsumer extends Component<any> {
-  static contextTypes = {
+  static contextTypes: {
+    array: any,
+    bool: any,
+    func: any,
+    null: any,
+    number: any,
+    object: any,
+    string: any,
+    symbol: any,
+    undefined: any,
+  } = {
     array: PropTypes.array,
     bool: PropTypes.bool,
     func: PropTypes.func,
@@ -68,26 +100,27 @@ class LegacyContextConsumer extends Component<any> {
     undefined: PropTypes.any,
   };
 
-  render() {
+  render(): any {
     return formatContextForDisplay('LegacyContextConsumer', this.context);
   }
 }
 
 class LegacyContextProviderWithUpdates extends Component<any> {
-  constructor(props) {
+  constructor(props: any) {
     super(props);
     this.state = {type: 'desktop'};
   }
 
-  getChildContext() {
+  getChildContext(): {type: any} {
     return {type: this.state.type};
   }
 
+  // $FlowFixMe[missing-local-annot]
   handleChange = event => {
     this.setState({type: event.target.value});
   };
 
-  render() {
+  render(): any {
     return (
       <>
         <LegacyFunctionalContextConsumer />
@@ -103,7 +136,8 @@ LegacyContextProviderWithUpdates.childContextTypes = {
   type: PropTypes.string,
 };
 
-function LegacyFunctionalContextConsumer(props, context) {
+// $FlowFixMe[missing-local-annot]
+function LegacyFunctionalContextConsumer(props: any, context) {
   return formatContextForDisplay('LegacyFunctionContextConsumer', context.type);
 }
 LegacyFunctionalContextConsumer.contextTypes = {
@@ -130,9 +164,9 @@ const UndefinedContext = createContext(undefined);
 UndefinedContext.displayName = 'UndefinedContext';
 
 class ModernContextType extends Component<any> {
-  static contextType = ModernContext;
+  static contextType: ReactContext<void> = ModernContext;
 
-  render() {
+  render(): any {
     return formatContextForDisplay('ModernContextType', this.context);
   }
 }
@@ -171,7 +205,9 @@ function FunctionalContextConsumerWithContextUpdates() {
   const {string2, setString2} = useContext(StringContextWithUpdates2);
   const [state, setState] = useState('state');
 
+  // $FlowFixMe[missing-local-annot]
   const handleChange = e => setString(e.target.value);
+  // $FlowFixMe[missing-local-annot]
   const handleChange2 = e => setString2(e.target.value);
 
   return (
@@ -198,7 +234,7 @@ function FunctionalContextConsumerWithContextUpdates() {
 }
 
 class ModernClassContextProviderWithUpdates extends Component<any> {
-  constructor(props) {
+  constructor(props: any) {
     super(props);
     this.setString = string => {
       this.setState({string});
@@ -210,7 +246,7 @@ class ModernClassContextProviderWithUpdates extends Component<any> {
     };
   }
 
-  render() {
+  render(): any {
     return (
       <StringContextWithUpdates.Provider value={this.state}>
         <ModernClassContextConsumerWithUpdates />
@@ -220,7 +256,7 @@ class ModernClassContextProviderWithUpdates extends Component<any> {
 }
 
 class ModernClassContextConsumerWithUpdates extends Component<any> {
-  render() {
+  render(): any {
     return (
       <StringContextWithUpdates.Consumer>
         {({string, setString}) => (
diff --git a/packages/react-devtools-shell/src/app/InspectableElements/CustomHooks.js b/packages/react-devtools-shell/src/app/InspectableElements/CustomHooks.js
index 66920af0d791e..2775376340108 100644
--- a/packages/react-devtools-shell/src/app/InspectableElements/CustomHooks.js
+++ b/packages/react-devtools-shell/src/app/InspectableElements/CustomHooks.js
@@ -103,7 +103,7 @@ function FunctionWithHooks(props: any, ref: React$Ref<any>) {
 const MemoWithHooks = memo(FunctionWithHooks);
 const ForwardRefWithHooks = forwardRef(FunctionWithHooks);
 
-function wrapWithHoc(Component) {
+function wrapWithHoc(Component: (props: any, ref: React$Ref<any>) => any) {
   function Hoc() {
     return <Component />;
   }
@@ -127,7 +127,7 @@ export default function CustomHooks(): React.Node {
 }
 
 // Below copied from https://usehooks.com/
-function useDebounce(value, delay) {
+function useDebounce(value: number, delay: number) {
   // State and setters for debounced value
   const [debouncedValue, setDebouncedValue] = useState(value);
 
diff --git a/packages/react-devtools-shell/src/app/InspectableElements/CustomObject.js b/packages/react-devtools-shell/src/app/InspectableElements/CustomObject.js
index 9d03fbd717515..f4de359f499fe 100644
--- a/packages/react-devtools-shell/src/app/InspectableElements/CustomObject.js
+++ b/packages/react-devtools-shell/src/app/InspectableElements/CustomObject.js
@@ -11,7 +11,7 @@ import * as React from 'react';
 
 class Custom {
   _number = 42;
-  get number() {
+  get number(): number {
     return this._number;
   }
 }
diff --git a/packages/react-devtools-shell/src/app/SuspenseTree/index.js b/packages/react-devtools-shell/src/app/SuspenseTree/index.js
index 8713d4d0275aa..e5b46bc7c8904 100644
--- a/packages/react-devtools-shell/src/app/SuspenseTree/index.js
+++ b/packages/react-devtools-shell/src/app/SuspenseTree/index.js
@@ -29,6 +29,7 @@ function EmptySuspense() {
   return <Suspense />;
 }
 
+// $FlowFixMe[missing-local-annot]
 function PrimaryFallbackTest({initialSuspend}) {
   const [suspend, setSuspend] = useState(initialSuspend);
   const fallbackStep = useTestSequence('fallback', Fallback1, Fallback2);
@@ -51,7 +52,7 @@ function PrimaryFallbackTest({initialSuspend}) {
   );
 }
 
-function useTestSequence(label, T1, T2) {
+function useTestSequence(label: string, T1: any => any, T2: any => any) {
   const [step, setStep] = useState(0);
   const next = (
     <button onClick={() => setStep(s => (s + 1) % allSteps.length)}>
diff --git a/packages/react-devtools-shell/src/app/devtools.js b/packages/react-devtools-shell/src/app/devtools.js
index 5915275978a4b..13e6609617118 100644
--- a/packages/react-devtools-shell/src/app/devtools.js
+++ b/packages/react-devtools-shell/src/app/devtools.js
@@ -81,7 +81,7 @@ inject('dist/app-index.js', () => {
   });
 });
 
-function inject(sourcePath, callback) {
+function inject(sourcePath: string, callback: () => void) {
   const script = contentDocument.createElement('script');
   script.onload = callback;
   script.src = sourcePath;
diff --git a/packages/react-devtools-shell/src/app/index.js b/packages/react-devtools-shell/src/app/index.js
index 3ec70de067152..6568fa7d3f019 100644
--- a/packages/react-devtools-shell/src/app/index.js
+++ b/packages/react-devtools-shell/src/app/index.js
@@ -43,7 +43,7 @@ function createContainer() {
   return container;
 }
 
-function mountApp(App) {
+function mountApp(App: () => React$Node) {
   const container = createContainer();
 
   const root = createRoot(container);
@@ -52,6 +52,7 @@ function mountApp(App) {
   unmountFunctions.push(() => root.unmount());
 }
 
+// $FlowFixMe[missing-local-annot]
 function mountStrictApp(App) {
   function StrictRoot() {
     return createElement(App);
@@ -65,7 +66,7 @@ function mountStrictApp(App) {
   unmountFunctions.push(() => root.unmount());
 }
 
-function mountLegacyApp(App) {
+function mountLegacyApp(App: () => React$Node) {
   function LegacyRender() {
     return createElement(App);
   }
diff --git a/packages/react-devtools-shell/src/e2e-apps/ListApp.js b/packages/react-devtools-shell/src/e2e-apps/ListApp.js
index f365686ad139b..9cb17188b0ffd 100644
--- a/packages/react-devtools-shell/src/e2e-apps/ListApp.js
+++ b/packages/react-devtools-shell/src/e2e-apps/ListApp.js
@@ -43,6 +43,7 @@ function List() {
   );
 }
 
+// $FlowFixMe[missing-local-annot]
 function ListItem({label}) {
   return <li data-testname="ListItem">{label}</li>;
 }
diff --git a/packages/react-devtools-shell/src/e2e-apps/ListAppLegacy.js b/packages/react-devtools-shell/src/e2e-apps/ListAppLegacy.js
index 8d0bb694a968b..056bd7b2243be 100644
--- a/packages/react-devtools-shell/src/e2e-apps/ListAppLegacy.js
+++ b/packages/react-devtools-shell/src/e2e-apps/ListAppLegacy.js
@@ -14,7 +14,7 @@ export default function App(): React.Node {
 }
 
 class List extends React.Component {
-  constructor(props) {
+  constructor(props: any) {
     super(props);
     this.state = {
       items: ['one', 'two', 'three'],
@@ -28,7 +28,7 @@ class List extends React.Component {
     }
   };
 
-  render() {
+  render(): any {
     return (
       <div>
         <input
@@ -50,6 +50,7 @@ class List extends React.Component {
   }
 }
 
+// $FlowFixMe[missing-local-annot]
 function ListItem({label}) {
   return <li data-testname="ListItem">{label}</li>;
 }
diff --git a/packages/react-devtools-shell/src/e2e-regression/app-legacy.js b/packages/react-devtools-shell/src/e2e-regression/app-legacy.js
index abb65dc36a3bf..dedbed5e73f7a 100644
--- a/packages/react-devtools-shell/src/e2e-regression/app-legacy.js
+++ b/packages/react-devtools-shell/src/e2e-regression/app-legacy.js
@@ -9,7 +9,7 @@ import ListApp from '../e2e-apps/ListApp';
 import ListAppLegacy from '../e2e-apps/ListAppLegacy';
 const version = process.env.E2E_APP_REACT_VERSION;
 
-function mountApp(App) {
+function mountApp(App: () => React$Node) {
   const container = document.createElement('div');
 
   ((document.body: any): HTMLBodyElement).appendChild(container);
diff --git a/packages/react-devtools-shell/src/e2e-regression/app.js b/packages/react-devtools-shell/src/e2e-regression/app.js
index d97f1a3f774d3..a2b4eb735cced 100644
--- a/packages/react-devtools-shell/src/e2e-regression/app.js
+++ b/packages/react-devtools-shell/src/e2e-regression/app.js
@@ -7,7 +7,7 @@ import * as ReactDOM from 'react-dom';
 import {createRoot} from 'react-dom/client';
 import ListApp from '../e2e-apps/ListApp';
 
-function mountApp(App) {
+function mountApp(App: () => React$Node) {
   const container = document.createElement('div');
 
   ((document.body: any): HTMLBodyElement).appendChild(container);
diff --git a/packages/react-devtools-shell/src/perf-regression/apps/LargeSubtree.js b/packages/react-devtools-shell/src/perf-regression/apps/LargeSubtree.js
index 63c37a6453be9..bee1c37ecaf56 100644
--- a/packages/react-devtools-shell/src/perf-regression/apps/LargeSubtree.js
+++ b/packages/react-devtools-shell/src/perf-regression/apps/LargeSubtree.js
@@ -9,7 +9,7 @@
 
 import * as React from 'react';
 
-function generateArray(size) {
+function generateArray(size: number) {
   return Array.from({length: size}, () => Math.floor(Math.random() * size));
 }
 
diff --git a/packages/react-devtools-timeline/src/EventTooltip.js b/packages/react-devtools-timeline/src/EventTooltip.js
index d5e4550a44225..ba3685f646fb2 100644
--- a/packages/react-devtools-timeline/src/EventTooltip.js
+++ b/packages/react-devtools-timeline/src/EventTooltip.js
@@ -15,11 +15,12 @@ import type {
   ReactComponentMeasure,
   ReactEventInfo,
   ReactMeasure,
-  TimelineData,
+  ReactMeasureType,
   SchedulingEvent,
   Snapshot,
   SuspenseEvent,
   ThrownError,
+  TimelineData,
   UserTimingMark,
 } from './types';
 
@@ -45,7 +46,7 @@ type Props = {
   width: number,
 };
 
-function getReactMeasureLabel(type): string | null {
+function getReactMeasureLabel(type: ReactMeasureType): string | null {
   switch (type) {
     case 'commit':
       return 'react commit';
diff --git a/packages/react-devtools-timeline/src/Timeline.js b/packages/react-devtools-timeline/src/Timeline.js
index 5ed6a0f7443f8..492725e17aa97 100644
--- a/packages/react-devtools-timeline/src/Timeline.js
+++ b/packages/react-devtools-timeline/src/Timeline.js
@@ -117,6 +117,7 @@ const ProcessingData = () => (
   </div>
 );
 
+// $FlowFixMe[missing-local-annot]
 const CouldNotLoadProfile = ({error, onFileSelect}) => (
   <div className={styles.EmptyStateContainer}>
     <div className={styles.Header}>Could not load profile</div>
diff --git a/packages/react-devtools-timeline/src/TimelineSearchInput.js b/packages/react-devtools-timeline/src/TimelineSearchInput.js
index 2f794a229639d..c448bded2a3a1 100644
--- a/packages/react-devtools-timeline/src/TimelineSearchInput.js
+++ b/packages/react-devtools-timeline/src/TimelineSearchInput.js
@@ -26,7 +26,8 @@ export default function TimelineSearchInput(props: Props): React.Node {
     return null;
   }
 
-  const search = text => dispatch({type: 'SET_SEARCH_TEXT', payload: text});
+  const search = (text: string) =>
+    dispatch({type: 'SET_SEARCH_TEXT', payload: text});
   const goToNextResult = () => dispatch({type: 'GO_TO_NEXT_SEARCH_RESULT'});
   const goToPreviousResult = () =>
     dispatch({type: 'GO_TO_PREVIOUS_SEARCH_RESULT'});
diff --git a/packages/react-devtools-timeline/src/content-views/ReactMeasuresView.js b/packages/react-devtools-timeline/src/content-views/ReactMeasuresView.js
index 27d65c55fa40c..8c6e4754ff1da 100644
--- a/packages/react-devtools-timeline/src/content-views/ReactMeasuresView.js
+++ b/packages/react-devtools-timeline/src/content-views/ReactMeasuresView.js
@@ -95,7 +95,7 @@ export class ReactMeasuresView extends View {
     scaleFactor: number,
     showGroupHighlight: boolean,
     showHoverHighlight: boolean,
-  ) {
+  ): void {
     const {frame, visibleArea} = this;
     const {timestamp, type, duration} = measure;
 
@@ -202,7 +202,7 @@ export class ReactMeasuresView extends View {
     }
   }
 
-  draw(context: CanvasRenderingContext2D) {
+  draw(context: CanvasRenderingContext2D): void {
     const {
       frame,
       _hoveredMeasure,
diff --git a/packages/react-devtools-timeline/src/view-base/Surface.js b/packages/react-devtools-timeline/src/view-base/Surface.js
index 67cc5c2b8b70b..4708cd95f787a 100644
--- a/packages/react-devtools-timeline/src/view-base/Surface.js
+++ b/packages/react-devtools-timeline/src/view-base/Surface.js
@@ -22,7 +22,11 @@ export type ViewRefs = {
 };
 
 // hidpi canvas: https://www.html5rocks.com/en/tutorials/canvas/hidpi/
-function configureRetinaCanvas(canvas, height, width) {
+function configureRetinaCanvas(
+  canvas: HTMLCanvasElement,
+  height: number,
+  width: number,
+) {
   canvas.width = width * DPR;
   canvas.height = height * DPR;
   canvas.style.width = `${width}px`;
diff --git a/packages/react-dom-bindings/src/client/ReactDOMComponent.js b/packages/react-dom-bindings/src/client/ReactDOMComponent.js
index c1a61a81dc9da..13a41337f4c71 100644
--- a/packages/react-dom-bindings/src/client/ReactDOMComponent.js
+++ b/packages/react-dom-bindings/src/client/ReactDOMComponent.js
@@ -115,7 +115,7 @@ if (__DEV__) {
     webview: true,
   };
 
-  validatePropertiesInDevelopment = function(type, props) {
+  validatePropertiesInDevelopment = function(type: string, props: any) {
     validateARIAProperties(type, props);
     validateInputProperties(type, props);
     validateUnknownProperties(type, props, {
@@ -172,7 +172,10 @@ if (__DEV__) {
     console.error('Extra attributes from the server: %s', names);
   };
 
-  warnForInvalidEventListener = function(registrationName, listener) {
+  warnForInvalidEventListener = function(
+    registrationName: string,
+    listener: any,
+  ) {
     if (listener === false) {
       console.error(
         'Expected `%s` listener to be a function, instead got `false`.\n\n' +
diff --git a/packages/react-dom-bindings/src/client/ReactDOMHostConfig.js b/packages/react-dom-bindings/src/client/ReactDOMHostConfig.js
index db42f2ed243a3..db3c6f1c5df22 100644
--- a/packages/react-dom-bindings/src/client/ReactDOMHostConfig.js
+++ b/packages/react-dom-bindings/src/client/ReactDOMHostConfig.js
@@ -445,7 +445,7 @@ export const scheduleMicrotask: any =
           .catch(handleErrorInNextTick)
     : scheduleTimeout; // TODO: Determine the best fallback here.
 
-function handleErrorInNextTick(error) {
+function handleErrorInNextTick(error: any) {
   setTimeout(() => {
     throw error;
   });
@@ -913,7 +913,7 @@ export function registerSuspenseInstanceRetry(
   instance._reactRetry = callback;
 }
 
-function getNextHydratable(node) {
+function getNextHydratable(node: ?Node) {
   // Skip non-hydratable nodes.
   for (; node != null; node = ((node: any): Node).nextSibling) {
     const nodeType = node.nodeType;
diff --git a/packages/react-dom-bindings/src/client/ReactDOMInput.js b/packages/react-dom-bindings/src/client/ReactDOMInput.js
index e53a1c1463119..d9b1a2e55cecb 100644
--- a/packages/react-dom-bindings/src/client/ReactDOMInput.js
+++ b/packages/react-dom-bindings/src/client/ReactDOMInput.js
@@ -37,7 +37,7 @@ let didWarnCheckedDefaultChecked = false;
 let didWarnControlledToUncontrolled = false;
 let didWarnUncontrolledToControlled = false;
 
-function isControlled(props) {
+function isControlled(props: any) {
   const usesChecked = props.type === 'checkbox' || props.type === 'radio';
   return usesChecked ? props.checked != null : props.value != null;
 }
@@ -351,7 +351,7 @@ export function restoreControlledState(element: Element, props: Object) {
   updateNamedCousins(node, props);
 }
 
-function updateNamedCousins(rootNode, props) {
+function updateNamedCousins(rootNode: InputWithWrapperState, props: any) {
   const name = props.name;
   if (props.type === 'radio' && name != null) {
     let queryRoot: Element = rootNode;
diff --git a/packages/react-dom-bindings/src/client/ReactDOMSelect.js b/packages/react-dom-bindings/src/client/ReactDOMSelect.js
index eef4cc05c4d7f..3ecc9974c637d 100644
--- a/packages/react-dom-bindings/src/client/ReactDOMSelect.js
+++ b/packages/react-dom-bindings/src/client/ReactDOMSelect.js
@@ -38,7 +38,7 @@ const valuePropNames = ['value', 'defaultValue'];
 /**
  * Validation function for `value` and `defaultValue`.
  */
-function checkSelectPropTypes(props) {
+function checkSelectPropTypes(props: any) {
   if (__DEV__) {
     checkControlledValueProps('select', props);
 
diff --git a/packages/react-dom-bindings/src/client/getNodeForCharacterOffset.js b/packages/react-dom-bindings/src/client/getNodeForCharacterOffset.js
index 151b3588e9494..360d8ab3c5dcd 100644
--- a/packages/react-dom-bindings/src/client/getNodeForCharacterOffset.js
+++ b/packages/react-dom-bindings/src/client/getNodeForCharacterOffset.js
@@ -15,7 +15,7 @@ import {TEXT_NODE} from '../shared/HTMLNodeType';
  * @param {DOMElement|DOMTextNode} node
  * @return {DOMElement|DOMTextNode}
  */
-function getLeafNode(node) {
+function getLeafNode(node: ?(Node | Element)) {
   while (node && node.firstChild) {
     node = node.firstChild;
   }
@@ -29,7 +29,7 @@ function getLeafNode(node) {
  * @param {DOMElement|DOMTextNode} node
  * @return {?DOMElement|DOMTextNode}
  */
-function getSiblingNode(node) {
+function getSiblingNode(node: ?(Node | Element)) {
   while (node) {
     if (node.nextSibling) {
       return node.nextSibling;
diff --git a/packages/react-dom-bindings/src/client/inputValueTracking.js b/packages/react-dom-bindings/src/client/inputValueTracking.js
index b21c463d1b042..139f16f390719 100644
--- a/packages/react-dom-bindings/src/client/inputValueTracking.js
+++ b/packages/react-dom-bindings/src/client/inputValueTracking.js
@@ -78,9 +78,12 @@ function trackValueOnNode(node: any): ?ValueTracker {
   const {get, set} = descriptor;
   Object.defineProperty(node, valueField, {
     configurable: true,
+    // $FlowFixMe[missing-this-annot]
     get: function() {
       return get.call(this);
     },
+    // $FlowFixMe[missing-local-annot]
+    // $FlowFixMe[missing-this-annot]
     set: function(value) {
       if (__DEV__) {
         checkFormFieldValueStringCoercion(value);
@@ -101,7 +104,7 @@ function trackValueOnNode(node: any): ?ValueTracker {
     getValue() {
       return currentValue;
     },
-    setValue(value) {
+    setValue(value: string) {
       if (__DEV__) {
         checkFormFieldValueStringCoercion(value);
       }
diff --git a/packages/react-dom-bindings/src/events/DOMEventProperties.js b/packages/react-dom-bindings/src/events/DOMEventProperties.js
index ddae752fd7f5c..69f0df205270f 100644
--- a/packages/react-dom-bindings/src/events/DOMEventProperties.js
+++ b/packages/react-dom-bindings/src/events/DOMEventProperties.js
@@ -111,7 +111,7 @@ if (enableCreateEventHandleAPI) {
   topLevelEventsToReactNames.set('afterblur', null);
 }
 
-function registerSimpleEvent(domEventName, reactName) {
+function registerSimpleEvent(domEventName: DOMEventName, reactName: string) {
   topLevelEventsToReactNames.set(domEventName, reactName);
   registerTwoPhaseEvent(reactName, [domEventName]);
 }
diff --git a/packages/react-dom-bindings/src/events/DOMPluginEventSystem.js b/packages/react-dom-bindings/src/events/DOMPluginEventSystem.js
index d422a0e26c583..4011636dcaab0 100644
--- a/packages/react-dom-bindings/src/events/DOMPluginEventSystem.js
+++ b/packages/react-dom-bindings/src/events/DOMPluginEventSystem.js
@@ -462,6 +462,7 @@ function addTrappedEventListener(
   // need support for such browsers.
   if (enableLegacyFBSupport && isDeferredListenerForLegacyFBSupport) {
     const originalListener = listener;
+    // $FlowFixMe[missing-this-annot]
     listener = function(...p) {
       removeEventListener(
         targetContainer,
diff --git a/packages/react-dom-bindings/src/events/ReactDOMEventListener.js b/packages/react-dom-bindings/src/events/ReactDOMEventListener.js
index 2eca510571f12..487420be00917 100644
--- a/packages/react-dom-bindings/src/events/ReactDOMEventListener.js
+++ b/packages/react-dom-bindings/src/events/ReactDOMEventListener.js
@@ -112,10 +112,10 @@ export function createEventListenerWrapperWithPriority(
 }
 
 function dispatchDiscreteEvent(
-  domEventName,
-  eventSystemFlags,
-  container,
-  nativeEvent,
+  domEventName: DOMEventName,
+  eventSystemFlags: EventSystemFlags,
+  container: EventTarget,
+  nativeEvent: AnyNativeEvent,
 ) {
   const previousPriority = getCurrentUpdatePriority();
   const prevTransition = ReactCurrentBatchConfig.transition;
@@ -130,10 +130,10 @@ function dispatchDiscreteEvent(
 }
 
 function dispatchContinuousEvent(
-  domEventName,
-  eventSystemFlags,
-  container,
-  nativeEvent,
+  domEventName: DOMEventName,
+  eventSystemFlags: EventSystemFlags,
+  container: EventTarget,
+  nativeEvent: AnyNativeEvent,
 ) {
   const previousPriority = getCurrentUpdatePriority();
   const prevTransition = ReactCurrentBatchConfig.transition;
diff --git a/packages/react-dom-bindings/src/events/ReactDOMEventReplaying.js b/packages/react-dom-bindings/src/events/ReactDOMEventReplaying.js
index a10524fdb4966..8b61d7607024d 100644
--- a/packages/react-dom-bindings/src/events/ReactDOMEventReplaying.js
+++ b/packages/react-dom-bindings/src/events/ReactDOMEventReplaying.js
@@ -619,7 +619,7 @@ export function retryIfBlockedOn(
   if (queuedMouse !== null) {
     scheduleCallbackIfUnblocked(queuedMouse, unblocked);
   }
-  const unblock = queuedEvent =>
+  const unblock = (queuedEvent: QueuedReplayableEvent) =>
     scheduleCallbackIfUnblocked(queuedEvent, unblocked);
   queuedPointers.forEach(unblock);
   queuedPointerCaptures.forEach(unblock);
diff --git a/packages/react-dom-bindings/src/events/SyntheticEvent.js b/packages/react-dom-bindings/src/events/SyntheticEvent.js
index 763e14ce57e94..c406cd0fc3933 100644
--- a/packages/react-dom-bindings/src/events/SyntheticEvent.js
+++ b/packages/react-dom-bindings/src/events/SyntheticEvent.js
@@ -42,6 +42,7 @@ function createSyntheticEvent(Interface: EventInterfaceType) {
    * normalizing browser quirks. Subclasses do not necessarily have to implement a
    * DOM interface; custom application-specific events can also subclass this.
    */
+  // $FlowFixMe[missing-this-annot]
   function SyntheticBaseEvent(
     reactName: string | null,
     reactEventType: string,
@@ -83,6 +84,7 @@ function createSyntheticEvent(Interface: EventInterfaceType) {
 
   // $FlowFixMe[prop-missing] found when upgrading Flow
   assign(SyntheticBaseEvent.prototype, {
+    // $FlowFixMe[missing-this-annot]
     preventDefault: function() {
       this.defaultPrevented = true;
       const event = this.nativeEvent;
@@ -99,6 +101,7 @@ function createSyntheticEvent(Interface: EventInterfaceType) {
       this.isDefaultPrevented = functionThatReturnsTrue;
     },
 
+    // $FlowFixMe[missing-this-annot]
     stopPropagation: function() {
       const event = this.nativeEvent;
       if (!event) {
@@ -147,7 +150,7 @@ const EventInterface = {
   eventPhase: 0,
   bubbles: 0,
   cancelable: 0,
-  timeStamp: function(event) {
+  timeStamp: function(event: {[propName: string]: mixed}) {
     return event.timeStamp || Date.now();
   },
   defaultPrevented: 0,
@@ -168,7 +171,7 @@ let lastMovementX;
 let lastMovementY;
 let lastMouseEvent;
 
-function updateMouseMovementPolyfillState(event) {
+function updateMouseMovementPolyfillState(event: {[propName: string]: mixed}) {
   if (event !== lastMouseEvent) {
     if (lastMouseEvent && event.type === 'mousemove') {
       // $FlowFixMe assuming this is a number
@@ -373,7 +376,7 @@ const translateToKey = {
  * @param {object} nativeEvent Native browser event.
  * @return {string} Normalized `key` property.
  */
-function getEventKey(nativeEvent) {
+function getEventKey(nativeEvent: {[propName: string]: mixed}) {
   if (nativeEvent.key) {
     // Normalize inconsistent values reported by browsers due to
     // implementations of a working draft specification.
@@ -422,6 +425,8 @@ const modifierKeyToProp = {
 // Older browsers (Safari <= 10, iOS Safari <= 10.2) do not support
 // getModifierState. If getModifierState is not supported, we map it to a set of
 // modifier keys exposed by the event. In this case, Lock-keys are not supported.
+// $FlowFixMe[missing-local-annot]
+// $FlowFixMe[missing-this-annot]
 function modifierStateGetter(keyArg) {
   const syntheticEvent = this;
   const nativeEvent = syntheticEvent.nativeEvent;
@@ -432,7 +437,7 @@ function modifierStateGetter(keyArg) {
   return keyProp ? !!nativeEvent[keyProp] : false;
 }
 
-function getEventModifierState(nativeEvent) {
+function getEventModifierState(nativeEvent: {[propName: string]: mixed}) {
   return modifierStateGetter;
 }
 
@@ -453,7 +458,7 @@ const KeyboardEventInterface = {
   locale: 0,
   getModifierState: getEventModifierState,
   // Legacy Interface
-  charCode: function(event) {
+  charCode: function(event: {[propName: string]: mixed}) {
     // `charCode` is the result of a KeyPress event and represents the value of
     // the actual printable character.
 
@@ -467,7 +472,7 @@ const KeyboardEventInterface = {
     }
     return 0;
   },
-  keyCode: function(event) {
+  keyCode: function(event: {[propName: string]: mixed}) {
     // `keyCode` is the result of a KeyDown/Up event and represents the value of
     // physical keyboard key.
 
@@ -480,7 +485,7 @@ const KeyboardEventInterface = {
     }
     return 0;
   },
-  which: function(event) {
+  which: function(event: {[propName: string]: mixed}) {
     // `which` is an alias for either `keyCode` or `charCode` depending on the
     // type of the event.
     if (event.type === 'keypress') {
@@ -560,7 +565,7 @@ export const SyntheticTransitionEvent: $FlowFixMe = createSyntheticEvent(
  */
 const WheelEventInterface = {
   ...MouseEventInterface,
-  deltaX(event) {
+  deltaX(event: {[propName: string]: mixed}) {
     return 'deltaX' in event
       ? event.deltaX
       : // Fallback to `wheelDeltaX` for Webkit and normalize (right is positive).
@@ -569,7 +574,7 @@ const WheelEventInterface = {
         -event.wheelDeltaX
       : 0;
   },
-  deltaY(event) {
+  deltaY(event: {[propName: string]: mixed}) {
     return 'deltaY' in event
       ? event.deltaY
       : // Fallback to `wheelDeltaY` for Webkit and normalize (down is positive).
diff --git a/packages/react-dom-bindings/src/events/plugins/BeforeInputEventPlugin.js b/packages/react-dom-bindings/src/events/plugins/BeforeInputEventPlugin.js
index 0753fc1970252..96431e9fe1519 100644
--- a/packages/react-dom-bindings/src/events/plugins/BeforeInputEventPlugin.js
+++ b/packages/react-dom-bindings/src/events/plugins/BeforeInputEventPlugin.js
@@ -192,11 +192,11 @@ let isComposing = false;
  * @return {?object} A SyntheticCompositionEvent.
  */
 function extractCompositionEvent(
-  dispatchQueue,
-  domEventName,
-  targetInst,
-  nativeEvent,
-  nativeEventTarget,
+  dispatchQueue: DispatchQueue,
+  domEventName: DOMEventName,
+  targetInst: null | Fiber,
+  nativeEvent: AnyNativeEvent,
+  nativeEventTarget: null | EventTarget,
 ) {
   let eventType;
   let fallbackData;
@@ -379,11 +379,11 @@ function getFallbackBeforeInputChars(
  * @return {?object} A SyntheticInputEvent.
  */
 function extractBeforeInputEvent(
-  dispatchQueue,
-  domEventName,
-  targetInst,
-  nativeEvent,
-  nativeEventTarget,
+  dispatchQueue: DispatchQueue,
+  domEventName: DOMEventName,
+  targetInst: null | Fiber,
+  nativeEvent: AnyNativeEvent,
+  nativeEventTarget: null | EventTarget,
 ) {
   let chars;
 
diff --git a/packages/react-dom-bindings/src/events/plugins/ChangeEventPlugin.js b/packages/react-dom-bindings/src/events/plugins/ChangeEventPlugin.js
index a482eb9f663e1..cbfc872770d28 100644
--- a/packages/react-dom-bindings/src/events/plugins/ChangeEventPlugin.js
+++ b/packages/react-dom-bindings/src/events/plugins/ChangeEventPlugin.js
@@ -6,6 +6,7 @@
  *
  * @flow
  */
+import type {TextInstance, Instance} from '../../client/ReactDOMHostConfig';
 import type {AnyNativeEvent} from '../PluginModuleType';
 import type {DOMEventName} from '../DOMEventNames';
 import type {DispatchQueue} from '../DOMPluginEventSystem';
@@ -50,10 +51,11 @@ function registerEvents() {
 }
 
 function createAndAccumulateChangeEvent(
+  // $FlowFixMe[missing-local-annot]
   dispatchQueue,
-  inst,
-  nativeEvent,
-  target,
+  inst: null | Fiber,
+  nativeEvent: AnyNativeEvent,
+  target: null | EventTarget,
 ) {
   // Flag this event loop as needing state restore.
   enqueueStateRestore(((target: any): Node));
@@ -78,7 +80,7 @@ let activeElementInst = null;
 /**
  * SECTION: handle `change` event
  */
-function shouldUseChangeEvent(elem) {
+function shouldUseChangeEvent(elem: Instance | TextInstance) {
   const nodeName = elem.nodeName && elem.nodeName.toLowerCase();
   return (
     nodeName === 'select' ||
@@ -86,7 +88,7 @@ function shouldUseChangeEvent(elem) {
   );
 }
 
-function manualDispatchChangeEvent(nativeEvent) {
+function manualDispatchChangeEvent(nativeEvent: AnyNativeEvent) {
   const dispatchQueue = [];
   createAndAccumulateChangeEvent(
     dispatchQueue,
@@ -109,7 +111,7 @@ function manualDispatchChangeEvent(nativeEvent) {
   batchedUpdates(runEventInBatch, dispatchQueue);
 }
 
-function runEventInBatch(dispatchQueue) {
+function runEventInBatch(dispatchQueue: DispatchQueue) {
   processDispatchQueue(dispatchQueue, 0);
 }
 
@@ -120,7 +122,10 @@ function getInstIfValueChanged(targetInst: Object) {
   }
 }
 
-function getTargetInstForChangeEvent(domEventName: DOMEventName, targetInst) {
+function getTargetInstForChangeEvent(
+  domEventName: DOMEventName,
+  targetInst: null | Fiber,
+) {
   if (domEventName === 'change') {
     return targetInst;
   }
@@ -143,7 +148,10 @@ if (canUseDOM) {
  * and override the value property so that we can distinguish user events from
  * value changes in JS.
  */
-function startWatchingForValueChange(target, targetInst) {
+function startWatchingForValueChange(
+  target: Instance | TextInstance,
+  targetInst: null | Fiber,
+) {
   activeElement = target;
   activeElementInst = targetInst;
   (activeElement: any).attachEvent('onpropertychange', handlePropertyChange);
@@ -166,6 +174,7 @@ function stopWatchingForValueChange() {
  * (For IE <=9) Handles a propertychange event, sending a `change` event if
  * the value of the active element has changed.
  */
+// $FlowFixMe[missing-local-annot]
 function handlePropertyChange(nativeEvent) {
   if (nativeEvent.propertyName !== 'value') {
     return;
@@ -177,8 +186,8 @@ function handlePropertyChange(nativeEvent) {
 
 function handleEventsForInputEventPolyfill(
   domEventName: DOMEventName,
-  target,
-  targetInst,
+  target: Instance | TextInstance,
+  targetInst: null | Fiber,
 ) {
   if (domEventName === 'focusin') {
     // In IE9, propertychange fires for most input events but is buggy and
@@ -201,7 +210,7 @@ function handleEventsForInputEventPolyfill(
 // For IE8 and IE9.
 function getTargetInstForInputEventPolyfill(
   domEventName: DOMEventName,
-  targetInst,
+  targetInst: null | Fiber,
 ) {
   if (
     domEventName === 'selectionchange' ||
@@ -225,7 +234,7 @@ function getTargetInstForInputEventPolyfill(
 /**
  * SECTION: handle `click` event
  */
-function shouldUseClickEvent(elem) {
+function shouldUseClickEvent(elem: any) {
   // Use the `click` event to detect changes to checkbox and radio inputs.
   // This approach works across all browsers, whereas `change` does not fire
   // until `blur` in IE8.
@@ -237,7 +246,10 @@ function shouldUseClickEvent(elem) {
   );
 }
 
-function getTargetInstForClickEvent(domEventName: DOMEventName, targetInst) {
+function getTargetInstForClickEvent(
+  domEventName: DOMEventName,
+  targetInst: null | Fiber,
+) {
   if (domEventName === 'click') {
     return getInstIfValueChanged(targetInst);
   }
@@ -245,7 +257,7 @@ function getTargetInstForClickEvent(domEventName: DOMEventName, targetInst) {
 
 function getTargetInstForInputOrChangeEvent(
   domEventName: DOMEventName,
-  targetInst,
+  targetInst: null | Fiber,
 ) {
   if (domEventName === 'input' || domEventName === 'change') {
     return getInstIfValueChanged(targetInst);
diff --git a/packages/react-dom-bindings/src/events/plugins/SelectEventPlugin.js b/packages/react-dom-bindings/src/events/plugins/SelectEventPlugin.js
index 0fef17bc6a3c8..8a568aed3cfef 100644
--- a/packages/react-dom-bindings/src/events/plugins/SelectEventPlugin.js
+++ b/packages/react-dom-bindings/src/events/plugins/SelectEventPlugin.js
@@ -91,7 +91,11 @@ function getEventTargetDocument(eventTarget: any) {
  * @param {object} nativeEventTarget
  * @return {?SyntheticEvent}
  */
-function constructSelectEvent(dispatchQueue, nativeEvent, nativeEventTarget) {
+function constructSelectEvent(
+  dispatchQueue: DispatchQueue,
+  nativeEvent: AnyNativeEvent,
+  nativeEventTarget: null | EventTarget,
+) {
   // Ensure we have the right element, and that the user is not dragging a
   // selection (this matches native `select` event behavior). In HTML5, select
   // fires only on input and textarea thus if there's no focused element we
diff --git a/packages/react-dom-bindings/src/server/ReactDOMServerFormatConfig.js b/packages/react-dom-bindings/src/server/ReactDOMServerFormatConfig.js
index a7f1a20cc1f15..843c4bee1ff6d 100644
--- a/packages/react-dom-bindings/src/server/ReactDOMServerFormatConfig.js
+++ b/packages/react-dom-bindings/src/server/ReactDOMServerFormatConfig.js
@@ -155,15 +155,19 @@ const endAsyncScript = stringToPrecomputedChunk('" async=""></script>');
  * While untrusted script content should be made safe before using this api it will
  * ensure that the script cannot be early terminated or never terminated state
  */
-function escapeBootstrapScriptContent(scriptText) {
+function escapeBootstrapScriptContent(scriptText: string) {
   if (__DEV__) {
     checkHtmlStringCoercion(scriptText);
   }
   return ('' + scriptText).replace(scriptRegex, scriptReplacer);
 }
 const scriptRegex = /(<\/|<)(s)(cript)/gi;
-const scriptReplacer = (match, prefix, s, suffix) =>
-  `${prefix}${s === 's' ? '\\u0073' : '\\u0053'}${suffix}`;
+const scriptReplacer = (
+  match: string,
+  prefix: string,
+  s: string,
+  suffix: string,
+) => `${prefix}${s === 's' ? '\\u0073' : '\\u0053'}${suffix}`;
 
 export type BootstrapScriptDescriptor = {
   src: string,
@@ -737,8 +741,8 @@ const endOfStartTagSelfClosing = stringToPrecomputedChunk('/>');
 
 function pushInnerHTML(
   target: Array<Chunk | PrecomputedChunk>,
-  innerHTML,
-  children,
+  innerHTML: any,
+  children: any,
 ) {
   if (innerHTML != null) {
     if (children != null) {
@@ -775,7 +779,7 @@ let didWarnInvalidOptionChildren = false;
 let didWarnInvalidOptionInnerHTML = false;
 let didWarnSelectedSetOnOption = false;
 
-function checkSelectProp(props, propName) {
+function checkSelectProp(props: any, propName: string) {
   if (__DEV__) {
     const value = props[propName];
     if (value != null) {
@@ -2765,6 +2769,7 @@ export function writeInitialResources(
     const {src, integrity} = responseState.externalRuntimeConfig;
     preinitImpl(resources, src, {as: 'script', integrity});
   }
+  // $FlowFixMe[missing-local-annot]
   function flushLinkResource(resource) {
     if (!resource.flushed) {
       pushLinkImpl(target, resource.props, responseState);
@@ -2889,6 +2894,7 @@ export function writeImmediateResources(
   resources: Resources,
   responseState: ResponseState,
 ): boolean {
+  // $FlowFixMe[missing-local-annot]
   function flushLinkResource(resource) {
     if (!resource.flushed) {
       pushLinkImpl(target, resource.props, responseState);
diff --git a/packages/react-dom-bindings/src/server/escapeTextForBrowser.js b/packages/react-dom-bindings/src/server/escapeTextForBrowser.js
index 90407972ba02a..bccad868209dd 100644
--- a/packages/react-dom-bindings/src/server/escapeTextForBrowser.js
+++ b/packages/react-dom-bindings/src/server/escapeTextForBrowser.js
@@ -50,7 +50,7 @@ const matchHtmlRegExp = /["'&<>]/;
  * @public
  */
 
-function escapeHtml(string) {
+function escapeHtml(string: string) {
   if (__DEV__) {
     checkHtmlStringCoercion(string);
   }
diff --git a/packages/react-dom-bindings/src/shared/DOMProperty.js b/packages/react-dom-bindings/src/shared/DOMProperty.js
index 11b9e3341203f..7caa042296837 100644
--- a/packages/react-dom-bindings/src/shared/DOMProperty.js
+++ b/packages/react-dom-bindings/src/shared/DOMProperty.js
@@ -213,6 +213,7 @@ export function getPropertyInfo(name: string): PropertyInfo | null {
   return properties.hasOwnProperty(name) ? properties[name] : null;
 }
 
+// $FlowFixMe[missing-this-annot]
 function PropertyInfoRecord(
   name: string,
   type: PropertyType,
@@ -456,7 +457,7 @@ reservedProps.forEach(name => {
 });
 
 const CAMELIZE = /[\-\:]([a-z])/g;
-const capitalize = token => token[1].toUpperCase();
+const capitalize = (token: string) => token[1].toUpperCase();
 
 // This is a list of all SVG attributes that need special casing, namespacing,
 // or boolean value assignment. Regular attributes that just accept strings
diff --git a/packages/react-dom/src/client/ReactDOMRoot.js b/packages/react-dom/src/client/ReactDOMRoot.js
index a4b901d156d78..12f62e2f1c0b6 100644
--- a/packages/react-dom/src/client/ReactDOMRoot.js
+++ b/packages/react-dom/src/client/ReactDOMRoot.js
@@ -91,11 +91,13 @@ const defaultOnRecoverableError =
         console['error'](error);
       };
 
+// $FlowFixMe[missing-this-annot]
 function ReactDOMRoot(internalRoot: FiberRoot) {
   this._internalRoot = internalRoot;
 }
 
 // $FlowFixMe[prop-missing] found when upgrading Flow
+// $FlowFixMe[missing-this-annot]
 ReactDOMHydrationRoot.prototype.render = ReactDOMRoot.prototype.render = function(
   children: ReactNodeList,
 ): void {
@@ -146,6 +148,7 @@ ReactDOMHydrationRoot.prototype.render = ReactDOMRoot.prototype.render = functio
 };
 
 // $FlowFixMe[prop-missing] found when upgrading Flow
+// $FlowFixMe[missing-this-annot]
 ReactDOMHydrationRoot.prototype.unmount = ReactDOMRoot.prototype.unmount = function(): void {
   if (__DEV__) {
     if (typeof arguments[0] === 'function') {
@@ -259,6 +262,7 @@ export function createRoot(
   return new ReactDOMRoot(root);
 }
 
+// $FlowFixMe[missing-this-annot]
 function ReactDOMHydrationRoot(internalRoot: FiberRoot) {
   this._internalRoot = internalRoot;
 }
diff --git a/packages/react-dom/src/server/ReactDOMFizzServerNode.js b/packages/react-dom/src/server/ReactDOMFizzServerNode.js
index e739b61b89e4d..7fa6d6a9cc0b4 100644
--- a/packages/react-dom/src/server/ReactDOMFizzServerNode.js
+++ b/packages/react-dom/src/server/ReactDOMFizzServerNode.js
@@ -7,6 +7,7 @@
  * @flow
  */
 
+import type {Request} from 'react-server/src/ReactFizzServer';
 import type {ReactNodeList} from 'shared/ReactTypes';
 import type {Writable} from 'stream';
 import type {BootstrapScriptDescriptor} from 'react-dom-bindings/src/server/ReactDOMServerFormatConfig';
@@ -26,11 +27,11 @@ import {
   createRootFormatContext,
 } from 'react-dom-bindings/src/server/ReactDOMServerFormatConfig';
 
-function createDrainHandler(destination: Destination, request) {
+function createDrainHandler(destination: Destination, request: Request) {
   return () => startFlowing(request, destination);
 }
 
-function createAbortHandler(request, reason) {
+function createAbortHandler(request: Request, reason: string) {
   // eslint-disable-next-line react-internal/prod-error-codes
   return () => abort(request, new Error(reason));
 }
diff --git a/packages/react-dom/src/server/ReactDOMFizzStaticNode.js b/packages/react-dom/src/server/ReactDOMFizzStaticNode.js
index 5f1a6f57409b5..eb0228fa6d52c 100644
--- a/packages/react-dom/src/server/ReactDOMFizzStaticNode.js
+++ b/packages/react-dom/src/server/ReactDOMFizzStaticNode.js
@@ -42,7 +42,7 @@ type StaticResult = {
   prelude: Readable,
 };
 
-function createFakeWritable(readable): Writable {
+function createFakeWritable(readable: any): Writable {
   // The current host config expects a Writable so we create
   // a fake writable for now to push into the Readable.
   return ({
diff --git a/packages/react-dom/src/server/ReactDOMLegacyServerImpl.js b/packages/react-dom/src/server/ReactDOMLegacyServerImpl.js
index 9563adfeec6bd..2d9fed4a556f1 100644
--- a/packages/react-dom/src/server/ReactDOMLegacyServerImpl.js
+++ b/packages/react-dom/src/server/ReactDOMLegacyServerImpl.js
@@ -43,12 +43,14 @@ function renderToStringImpl(
   let fatalError = null;
   let result = '';
   const destination = {
+    // $FlowFixMe[missing-local-annot]
     push(chunk) {
       if (chunk !== null) {
         result += chunk;
       }
       return true;
     },
+    // $FlowFixMe[missing-local-annot]
     destroy(error) {
       didFatal = true;
       fatalError = error;
diff --git a/packages/react-dom/src/server/ReactDOMLegacyServerNodeStream.js b/packages/react-dom/src/server/ReactDOMLegacyServerNodeStream.js
index e0726fa79eb03..167d6cc1e1147 100644
--- a/packages/react-dom/src/server/ReactDOMLegacyServerNodeStream.js
+++ b/packages/react-dom/src/server/ReactDOMLegacyServerNodeStream.js
@@ -40,12 +40,14 @@ class ReactMarkupReadableStream extends Readable {
     this.startedFlowing = false;
   }
 
+  // $FlowFixMe[missing-local-annot]
   _destroy(err, callback) {
     abort(this.request);
     // $FlowFixMe: The type definition for the callback should allow undefined and null.
     callback(err);
   }
 
+  // $FlowFixMe[missing-local-annot]
   _read(size) {
     if (this.startedFlowing) {
       startFlowing(this.request, this);
diff --git a/packages/react-interactions/events/src/dom/create-event-handle/Focus.js b/packages/react-interactions/events/src/dom/create-event-handle/Focus.js
index 563fbaae0b474..58b4eb44f5092 100644
--- a/packages/react-interactions/events/src/dom/create-event-handle/Focus.js
+++ b/packages/react-interactions/events/src/dom/create-event-handle/Focus.js
@@ -97,7 +97,7 @@ function handleGlobalFocusVisibleEvent(
 
 function handleFocusVisibleTargetEvents(
   event: SyntheticEvent<EventTarget>,
-  callback,
+  callback: boolean => void,
 ): void {
   if (event.type === 'keydown') {
     const {nativeEvent} = (event: any);
@@ -125,9 +125,10 @@ function isRelatedTargetWithin(
 }
 
 function setFocusVisibleListeners(
+  // $FlowFixMe[missing-local-annot]
   focusVisibleHandles,
   focusTarget: EventTarget,
-  callback,
+  callback: boolean => void,
 ) {
   focusVisibleHandles.forEach(focusVisibleHandle => {
     focusVisibleHandle.setListener(focusTarget, event =>
diff --git a/packages/react-native-renderer/src/ReactFabric.js b/packages/react-native-renderer/src/ReactFabric.js
index 4ec730a3f9a5e..bcd04fb7596b3 100644
--- a/packages/react-native-renderer/src/ReactFabric.js
+++ b/packages/react-native-renderer/src/ReactFabric.js
@@ -201,6 +201,7 @@ function sendAccessibilityEvent(handle: any, eventType: string) {
   }
 }
 
+// $FlowFixMe[missing-local-annot]
 function onRecoverableError(error) {
   // TODO: Expose onRecoverableError option to userspace
   // eslint-disable-next-line react-internal/no-production-logging, react-internal/warning-args
@@ -236,6 +237,7 @@ function render(
   return getPublicRootInstance(root);
 }
 
+// $FlowFixMe[missing-this-annot]
 function unmountComponentAtNode(containerTag: number) {
   this.stopSurface(containerTag);
 }
diff --git a/packages/react-native-renderer/src/ReactFabricHostConfig.js b/packages/react-native-renderer/src/ReactFabricHostConfig.js
index ec6918e96d346..7f98be1c9f37b 100644
--- a/packages/react-native-renderer/src/ReactFabricHostConfig.js
+++ b/packages/react-native-renderer/src/ReactFabricHostConfig.js
@@ -244,6 +244,7 @@ class ReactFabricHostComponent {
     eventType: string,
     listener: EventListener,
     options: EventListenerOptions | boolean,
+    // $FlowFixMe[missing-local-annot]
   ) {
     if (typeof eventType !== 'string') {
       throw new Error('addEventListener_unstable eventType must be a string');
diff --git a/packages/react-native-renderer/src/ReactNativeBridgeEventPlugin.js b/packages/react-native-renderer/src/ReactNativeBridgeEventPlugin.js
index 618cdce27b644..3e7a3a91adf5f 100644
--- a/packages/react-native-renderer/src/ReactNativeBridgeEventPlugin.js
+++ b/packages/react-native-renderer/src/ReactNativeBridgeEventPlugin.js
@@ -31,12 +31,14 @@ const {
 // Start of inline: the below functions were inlined from
 // EventPropagator.js, as they deviated from ReactDOM's newer
 // implementations.
+// $FlowFixMe[missing-local-annot]
 function listenersAtPhase(inst, event, propagationPhase: PropagationPhases) {
   const registrationName =
     event.dispatchConfig.phasedRegistrationNames[propagationPhase];
   return getListeners(inst, registrationName, propagationPhase, true);
 }
 
+// $FlowFixMe[missing-local-annot]
 function accumulateListenersAndInstances(inst, event, listeners) {
   const listenersLength = listeners
     ? isArray(listeners)
@@ -64,6 +66,7 @@ function accumulateListenersAndInstances(inst, event, listeners) {
   }
 }
 
+// $FlowFixMe[missing-local-annot]
 function accumulateDirectionalDispatches(inst, phase, event) {
   if (__DEV__) {
     if (!inst) {
@@ -74,6 +77,7 @@ function accumulateDirectionalDispatches(inst, phase, event) {
   accumulateListenersAndInstances(inst, event, listeners);
 }
 
+// $FlowFixMe[missing-local-annot]
 function getParent(inst) {
   do {
     inst = inst.return;
@@ -117,6 +121,7 @@ export function traverseTwoPhase(
   }
 }
 
+// $FlowFixMe[missing-local-annot]
 function accumulateTwoPhaseDispatchesSingle(event) {
   if (event && event.dispatchConfig.phasedRegistrationNames) {
     traverseTwoPhase(
@@ -128,10 +133,12 @@ function accumulateTwoPhaseDispatchesSingle(event) {
   }
 }
 
+// $FlowFixMe[missing-local-annot]
 function accumulateTwoPhaseDispatches(events) {
   forEachAccumulated(events, accumulateTwoPhaseDispatchesSingle);
 }
 
+// $FlowFixMe[missing-local-annot]
 function accumulateCapturePhaseDispatches(event) {
   if (event && event.dispatchConfig.phasedRegistrationNames) {
     traverseTwoPhase(
diff --git a/packages/react-native-renderer/src/ReactNativeEventEmitter.js b/packages/react-native-renderer/src/ReactNativeEventEmitter.js
index aeb3e23cb0368..f4ed0f641623b 100644
--- a/packages/react-native-renderer/src/ReactNativeEventEmitter.js
+++ b/packages/react-native-renderer/src/ReactNativeEventEmitter.js
@@ -43,6 +43,7 @@ const EMPTY_NATIVE_EVENT = (({}: any): AnyNativeEvent);
  * @param {Array<number>} indices Indices by which to pull subsequence.
  * @return {Array<Touch>} Subsequence of touch objects.
  */
+// $FlowFixMe[missing-local-annot]
 const touchSubsequence = function(touches, indices) {
   const ret = [];
   for (let i = 0; i < indices.length; i++) {
diff --git a/packages/react-native-renderer/src/ReactNativeFiberInspector.js b/packages/react-native-renderer/src/ReactNativeFiberInspector.js
index 14545550647dd..01d71a2825cb4 100644
--- a/packages/react-native-renderer/src/ReactNativeFiberInspector.js
+++ b/packages/react-native-renderer/src/ReactNativeFiberInspector.js
@@ -39,6 +39,7 @@ let getOwnerHierarchy;
 let traverseOwnerTreeUp;
 
 if (__DEV__ || enableGetInspectorDataForInstanceInProduction) {
+  // $FlowFixMe[missing-local-annot]
   createHierarchy = function(fiberHierarchy) {
     return fiberHierarchy.map(fiber => ({
       name: getComponentNameFromType(fiber.type),
@@ -68,6 +69,7 @@ if (__DEV__ || enableGetInspectorDataForInstanceInProduction) {
     }));
   };
 
+  // $FlowFixMe[missing-local-annot]
   getHostNode = function(fiber: Fiber | null, findNodeHandle) {
     let hostNode;
     // look for children first for the hostNode
@@ -84,6 +86,7 @@ if (__DEV__ || enableGetInspectorDataForInstanceInProduction) {
     return null;
   };
 
+  // $FlowFixMe[missing-local-annot]
   getHostProps = function(fiber) {
     const host = findCurrentHostFiber(fiber);
     if (host) {
@@ -128,6 +131,7 @@ if (__DEV__ || enableGetInspectorDataForInstanceInProduction) {
     return hierarchy;
   };
 
+  // $FlowFixMe[missing-local-annot]
   lastNonHostInstance = function(hierarchy) {
     for (let i = hierarchy.length - 1; i > 1; i--) {
       const instance = hierarchy[i];
@@ -139,6 +143,7 @@ if (__DEV__ || enableGetInspectorDataForInstanceInProduction) {
     return hierarchy[0];
   };
 
+  // $FlowFixMe[missing-local-annot]
   traverseOwnerTreeUp = function(hierarchy, instance: any) {
     if (instance) {
       hierarchy.unshift(instance);
diff --git a/packages/react-native-renderer/src/ReactNativeGetListeners.js b/packages/react-native-renderer/src/ReactNativeGetListeners.js
index 3417f69b4a3c6..2f3520a1ceb22 100644
--- a/packages/react-native-renderer/src/ReactNativeGetListeners.js
+++ b/packages/react-native-renderer/src/ReactNativeGetListeners.js
@@ -117,6 +117,7 @@ export default function getListeners(
       // all imperative event listeners in a function to unwrap the SyntheticEvent
       // and pass them an Event.
       // When this API is more stable and used more frequently, we can revisit.
+      // $FlowFixMe[missing-local-annot]
       const listenerFnWrapper = function(syntheticEvent, ...args) {
         const eventInst = new CustomEvent(mangledImperativeRegistrationName, {
           detail: syntheticEvent.nativeEvent,
diff --git a/packages/react-native-renderer/src/ReactNativeRenderer.js b/packages/react-native-renderer/src/ReactNativeRenderer.js
index 85e8a1436a9f4..e07b8285a4d4a 100644
--- a/packages/react-native-renderer/src/ReactNativeRenderer.js
+++ b/packages/react-native-renderer/src/ReactNativeRenderer.js
@@ -198,6 +198,7 @@ function sendAccessibilityEvent(handle: any, eventType: string) {
   }
 }
 
+// $FlowFixMe[missing-local-annot]
 function onRecoverableError(error) {
   // TODO: Expose onRecoverableError option to userspace
   // eslint-disable-next-line react-internal/no-production-logging, react-internal/warning-args
diff --git a/packages/react-native-renderer/src/legacy-events/EventBatching.js b/packages/react-native-renderer/src/legacy-events/EventBatching.js
index 26bb977492811..ec98a86fb61df 100644
--- a/packages/react-native-renderer/src/legacy-events/EventBatching.js
+++ b/packages/react-native-renderer/src/legacy-events/EventBatching.js
@@ -34,6 +34,7 @@ const executeDispatchesAndRelease = function(event: ReactSyntheticEvent) {
     }
   }
 };
+// $FlowFixMe[missing-local-annot]
 const executeDispatchesAndReleaseTopLevel = function(e) {
   return executeDispatchesAndRelease(e);
 };
diff --git a/packages/react-reconciler/src/DebugTracing.js b/packages/react-reconciler/src/DebugTracing.js
index e2162892fd7f6..a5302f3dcebde 100644
--- a/packages/react-reconciler/src/DebugTracing.js
+++ b/packages/react-reconciler/src/DebugTracing.js
@@ -22,7 +22,7 @@ function formatLanes(laneOrLanes: Lane | Lanes): string {
   return '0b' + (laneOrLanes: any).toString(2).padStart(31, '0');
 }
 
-function group(...groupArgs): void {
+function group(...groupArgs: Array<string>): void {
   pendingGroupArgs.push(groupArgs);
 
   if (nativeConsoleLog === null) {
@@ -44,7 +44,7 @@ function groupEnd(): void {
   }
 }
 
-function log(...logArgs): void {
+function log(...logArgs: Array<mixed>): void {
   if (printedGroupIndex < pendingGroupArgs.length - 1) {
     for (let i = printedGroupIndex + 1; i < pendingGroupArgs.length; i++) {
       const groupArgs = pendingGroupArgs[i];
diff --git a/packages/react-reconciler/src/ReactChildFiber.js b/packages/react-reconciler/src/ReactChildFiber.js
index b3d2fe0239f8b..09f318969a001 100644
--- a/packages/react-reconciler/src/ReactChildFiber.js
+++ b/packages/react-reconciler/src/ReactChildFiber.js
@@ -97,7 +97,7 @@ if (__DEV__) {
   };
 }
 
-function isReactClass(type) {
+function isReactClass(type: any) {
   return type.prototype && type.prototype.isReactComponent;
 }
 
@@ -203,7 +203,7 @@ function coerceRef(
       ) {
         return current.ref;
       }
-      const ref = function(value) {
+      const ref = function(value: mixed) {
         const refs = resolvedInst.refs;
         if (value === null) {
           delete refs[stringRef];
@@ -267,7 +267,7 @@ function warnOnFunctionType(returnFiber: Fiber) {
   }
 }
 
-function resolveLazy(lazyType) {
+function resolveLazy(lazyType: any) {
   const payload = lazyType._payload;
   const init = lazyType._init;
   return init(payload);
@@ -284,7 +284,9 @@ type ChildReconciler = (
 // to be able to optimize each path individually by branching early. This needs
 // a compiler or we can do it manually. Helpers that don't need this branching
 // live outside of this function.
-function createChildReconciler(shouldTrackSideEffects): ChildReconciler {
+function createChildReconciler(
+  shouldTrackSideEffects: boolean,
+): ChildReconciler {
   function deleteChild(returnFiber: Fiber, childToDelete: Fiber): void {
     if (!shouldTrackSideEffects) {
       // Noop.
diff --git a/packages/react-reconciler/src/ReactFiber.js b/packages/react-reconciler/src/ReactFiber.js
index f1e473ebf0b98..44e21bc4ecd65 100644
--- a/packages/react-reconciler/src/ReactFiber.js
+++ b/packages/react-reconciler/src/ReactFiber.js
@@ -132,6 +132,7 @@ if (__DEV__) {
 }
 
 function FiberNode(
+  this: $FlowFixMe,
   tag: WorkTag,
   pendingProps: mixed,
   key: null | string,
diff --git a/packages/react-reconciler/src/ReactFiberBeginWork.js b/packages/react-reconciler/src/ReactFiberBeginWork.js
index 53ed65925b97f..cb48a75525349 100644
--- a/packages/react-reconciler/src/ReactFiberBeginWork.js
+++ b/packages/react-reconciler/src/ReactFiberBeginWork.js
@@ -1085,11 +1085,11 @@ function markRef(current: Fiber | null, workInProgress: Fiber) {
 }
 
 function updateFunctionComponent(
-  current,
-  workInProgress,
-  Component,
+  current: null | Fiber,
+  workInProgress: Fiber,
+  Component: any,
   nextProps: any,
-  renderLanes,
+  renderLanes: Lanes,
 ) {
   if (__DEV__) {
     if (workInProgress.type !== workInProgress.elementType) {
@@ -1425,7 +1425,7 @@ function finishClassComponent(
   return workInProgress.child;
 }
 
-function pushHostRootContext(workInProgress) {
+function pushHostRootContext(workInProgress: Fiber) {
   const root = (workInProgress.stateNode: FiberRoot);
   if (root.pendingContext) {
     pushTopLevelContextObject(
@@ -1440,7 +1440,11 @@ function pushHostRootContext(workInProgress) {
   pushHostContainer(workInProgress, root.containerInfo);
 }
 
-function updateHostRoot(current, workInProgress, renderLanes) {
+function updateHostRoot(
+  current: null | Fiber,
+  workInProgress: Fiber,
+  renderLanes: Lanes,
+) {
   pushHostRootContext(workInProgress);
 
   if (current === null) {
@@ -1625,7 +1629,11 @@ function updateHostComponent(
   return workInProgress.child;
 }
 
-function updateHostResource(current, workInProgress, renderLanes) {
+function updateHostResource(
+  current: null | Fiber,
+  workInProgress: Fiber,
+  renderLanes: Lanes,
+) {
   pushHostContext(workInProgress);
   markRef(current, workInProgress);
   const currentProps = current === null ? null : current.memoizedProps;
@@ -1673,7 +1681,7 @@ function updateHostSingleton(
   return workInProgress.child;
 }
 
-function updateHostText(current, workInProgress) {
+function updateHostText(current: null | Fiber, workInProgress: Fiber) {
   if (current === null) {
     tryToClaimNextHydratableInstance(workInProgress);
   }
@@ -1683,10 +1691,10 @@ function updateHostText(current, workInProgress) {
 }
 
 function mountLazyComponent(
-  _current,
-  workInProgress,
-  elementType,
-  renderLanes,
+  _current: null | Fiber,
+  workInProgress: Fiber,
+  elementType: any,
+  renderLanes: Lanes,
 ) {
   resetSuspendedCurrentOnMountInLegacyMode(_current, workInProgress);
 
@@ -1792,11 +1800,11 @@ function mountLazyComponent(
 }
 
 function mountIncompleteClassComponent(
-  _current,
-  workInProgress,
-  Component,
-  nextProps,
-  renderLanes,
+  _current: null | Fiber,
+  workInProgress: Fiber,
+  Component: any,
+  nextProps: any,
+  renderLanes: Lanes,
 ) {
   resetSuspendedCurrentOnMountInLegacyMode(_current, workInProgress);
 
@@ -1831,10 +1839,10 @@ function mountIncompleteClassComponent(
 }
 
 function mountIndeterminateComponent(
-  _current,
-  workInProgress,
-  Component,
-  renderLanes,
+  _current: null | Fiber,
+  workInProgress: Fiber,
+  Component: $FlowFixMe,
+  renderLanes: Lanes,
 ) {
   resetSuspendedCurrentOnMountInLegacyMode(_current, workInProgress);
 
@@ -2171,12 +2179,16 @@ function shouldRemainOnFallback(
   );
 }
 
-function getRemainingWorkInPrimaryTree(current: Fiber, renderLanes) {
+function getRemainingWorkInPrimaryTree(current: Fiber, renderLanes: Lanes) {
   // TODO: Should not remove render lanes that were pinged during this render
   return removeLanes(current.childLanes, renderLanes);
 }
 
-function updateSuspenseComponent(current, workInProgress, renderLanes) {
+function updateSuspenseComponent(
+  current: null | Fiber,
+  workInProgress: Fiber,
+  renderLanes: Lanes,
+) {
   const nextProps = workInProgress.pendingProps;
 
   // This is used by DevTools to force a boundary to suspend.
@@ -2423,9 +2435,9 @@ function updateSuspenseComponent(current, workInProgress, renderLanes) {
 }
 
 function mountSuspensePrimaryChildren(
-  workInProgress,
-  primaryChildren,
-  renderLanes,
+  workInProgress: Fiber,
+  primaryChildren: $FlowFixMe,
+  renderLanes: Lanes,
 ) {
   const mode = workInProgress.mode;
   const primaryChildProps: OffscreenProps = {
@@ -2443,10 +2455,10 @@ function mountSuspensePrimaryChildren(
 }
 
 function mountSuspenseFallbackChildren(
-  workInProgress,
-  primaryChildren,
-  fallbackChildren,
-  renderLanes,
+  workInProgress: Fiber,
+  primaryChildren: $FlowFixMe,
+  fallbackChildren: $FlowFixMe,
+  renderLanes: Lanes,
 ) {
   const mode = workInProgress.mode;
   const progressedPrimaryFragment: Fiber | null = workInProgress.child;
@@ -2526,10 +2538,10 @@ function updateWorkInProgressOffscreenFiber(
 }
 
 function updateSuspensePrimaryChildren(
-  current,
-  workInProgress,
-  primaryChildren,
-  renderLanes,
+  current: Fiber,
+  workInProgress: Fiber,
+  primaryChildren: $FlowFixMe,
+  renderLanes: Lanes,
 ) {
   const currentPrimaryChildFragment: Fiber = (current.child: any);
   const currentFallbackChildFragment: Fiber | null =
@@ -2563,11 +2575,11 @@ function updateSuspensePrimaryChildren(
 }
 
 function updateSuspenseFallbackChildren(
-  current,
-  workInProgress,
-  primaryChildren,
-  fallbackChildren,
-  renderLanes,
+  current: Fiber,
+  workInProgress: Fiber,
+  primaryChildren: $FlowFixMe,
+  fallbackChildren: $FlowFixMe,
+  renderLanes: Lanes,
 ) {
   const mode = workInProgress.mode;
   const currentPrimaryChildFragment: Fiber = (current.child: any);
@@ -2688,11 +2700,11 @@ function retrySuspenseComponentWithoutHydrating(
 }
 
 function mountSuspenseFallbackAfterRetryWithoutHydrating(
-  current,
-  workInProgress,
-  primaryChildren,
-  fallbackChildren,
-  renderLanes,
+  current: Fiber,
+  workInProgress: Fiber,
+  primaryChildren: $FlowFixMe,
+  fallbackChildren: $FlowFixMe,
+  renderLanes: Lanes,
 ) {
   const fiberMode = workInProgress.mode;
   const primaryChildProps: OffscreenProps = {
@@ -3550,7 +3562,11 @@ function updateContextConsumer(
   return workInProgress.child;
 }
 
-function updateScopeComponent(current, workInProgress, renderLanes) {
+function updateScopeComponent(
+  current: null | Fiber,
+  workInProgress: Fiber,
+  renderLanes: Lanes,
+) {
   const nextProps = workInProgress.pendingProps;
   const nextChildren = nextProps.children;
 
@@ -3566,7 +3582,10 @@ export function checkIfWorkInProgressReceivedUpdate(): boolean {
   return didReceiveUpdate;
 }
 
-function resetSuspendedCurrentOnMountInLegacyMode(current, workInProgress) {
+function resetSuspendedCurrentOnMountInLegacyMode(
+  current: null | Fiber,
+  workInProgress: Fiber,
+) {
   if ((workInProgress.mode & ConcurrentMode) === NoMode) {
     if (current !== null) {
       // A lazy component only mounts if it suspended inside a non-
diff --git a/packages/react-reconciler/src/ReactFiberCacheComponent.js b/packages/react-reconciler/src/ReactFiberCacheComponent.js
index 90a1e698a6ad1..08f3c34c09653 100644
--- a/packages/react-reconciler/src/ReactFiberCacheComponent.js
+++ b/packages/react-reconciler/src/ReactFiberCacheComponent.js
@@ -21,7 +21,8 @@ import * as Scheduler from 'scheduler';
 const AbortControllerLocal: typeof AbortController = enableCache
   ? typeof AbortController !== 'undefined'
     ? AbortController
-    : (function AbortControllerShim() {
+    : // $FlowFixMe[missing-this-annot]
+      (function AbortControllerShim() {
         const listeners = [];
         const signal = (this.signal = {
           aborted: false,
diff --git a/packages/react-reconciler/src/ReactFiberClassComponent.js b/packages/react-reconciler/src/ReactFiberClassComponent.js
index f8a80f7ffbeb1..2443db472ba1d 100644
--- a/packages/react-reconciler/src/ReactFiberClassComponent.js
+++ b/packages/react-reconciler/src/ReactFiberClassComponent.js
@@ -118,7 +118,7 @@ if (__DEV__) {
     }
   };
 
-  warnOnUndefinedDerivedState = function(type, partialState) {
+  warnOnUndefinedDerivedState = function(type: any, partialState: any) {
     if (partialState === undefined) {
       const componentName = getComponentNameFromType(type) || 'Component';
       if (!didWarnAboutUndefinedDerivedState.has(componentName)) {
@@ -194,7 +194,8 @@ function applyDerivedStateFromProps(
 
 const classComponentUpdater = {
   isMounted,
-  enqueueSetState(inst, payload, callback) {
+  // $FlowFixMe[missing-local-annot]
+  enqueueSetState(inst: any, payload: any, callback) {
     const fiber = getInstance(inst);
     const eventTime = requestEventTime();
     const lane = requestUpdateLane(fiber);
@@ -227,7 +228,7 @@ const classComponentUpdater = {
       markStateUpdateScheduled(fiber, lane);
     }
   },
-  enqueueReplaceState(inst, payload, callback) {
+  enqueueReplaceState(inst: any, payload: any, callback: null) {
     const fiber = getInstance(inst);
     const eventTime = requestEventTime();
     const lane = requestUpdateLane(fiber);
@@ -262,7 +263,8 @@ const classComponentUpdater = {
       markStateUpdateScheduled(fiber, lane);
     }
   },
-  enqueueForceUpdate(inst, callback) {
+  // $FlowFixMe[missing-local-annot]
+  enqueueForceUpdate(inst: any, callback) {
     const fiber = getInstance(inst);
     const eventTime = requestEventTime();
     const lane = requestUpdateLane(fiber);
@@ -299,13 +301,13 @@ const classComponentUpdater = {
 };
 
 function checkShouldComponentUpdate(
-  workInProgress,
-  ctor,
-  oldProps,
-  newProps,
-  oldState,
-  newState,
-  nextContext,
+  workInProgress: Fiber,
+  ctor: any,
+  oldProps: any,
+  newProps: any,
+  oldState: any,
+  newState: any,
+  nextContext: any,
 ) {
   const instance = workInProgress.stateNode;
   if (typeof instance.shouldComponentUpdate === 'function') {
@@ -760,7 +762,7 @@ function constructClassInstance(
   return instance;
 }
 
-function callComponentWillMount(workInProgress, instance) {
+function callComponentWillMount(workInProgress: Fiber, instance: any) {
   const oldState = instance.state;
 
   if (typeof instance.componentWillMount === 'function') {
@@ -784,10 +786,10 @@ function callComponentWillMount(workInProgress, instance) {
 }
 
 function callComponentWillReceiveProps(
-  workInProgress,
-  instance,
-  newProps,
-  nextContext,
+  workInProgress: Fiber,
+  instance: any,
+  newProps: any,
+  nextContext: any,
 ) {
   const oldState = instance.state;
   if (typeof instance.componentWillReceiveProps === 'function') {
diff --git a/packages/react-reconciler/src/ReactFiberClassUpdateQueue.js b/packages/react-reconciler/src/ReactFiberClassUpdateQueue.js
index fa2de881dd59a..9f740716edc30 100644
--- a/packages/react-reconciler/src/ReactFiberClassUpdateQueue.js
+++ b/packages/react-reconciler/src/ReactFiberClassUpdateQueue.js
@@ -676,7 +676,7 @@ export function processUpdateQueue<State>(
   }
 }
 
-function callCallback(callback, context) {
+function callCallback(callback: () => mixed, context: any) {
   if (typeof callback !== 'function') {
     throw new Error(
       'Invalid argument passed as callback. Expected a function. Instead ' +
diff --git a/packages/react-reconciler/src/ReactFiberCommitWork.js b/packages/react-reconciler/src/ReactFiberCommitWork.js
index 363e6408711cf..e58ecf95e0877 100644
--- a/packages/react-reconciler/src/ReactFiberCommitWork.js
+++ b/packages/react-reconciler/src/ReactFiberCommitWork.js
@@ -248,7 +248,10 @@ export function reportUncaughtErrorInDEV(error: mixed) {
   }
 }
 
-const callComponentWillUnmountWithTimer = function(current, instance) {
+const callComponentWillUnmountWithTimer = function(
+  current: Fiber,
+  instance: any,
+) {
   instance.props = current.memoizedProps;
   instance.state = current.memoizedState;
   if (shouldProfile(current)) {
@@ -1495,7 +1498,7 @@ function commitTransitionProgress(offscreenFiber: Fiber) {
   }
 }
 
-function hideOrUnhideAllChildren(finishedWork, isHidden) {
+function hideOrUnhideAllChildren(finishedWork: Fiber, isHidden: boolean) {
   // Only hide or unhide the top-most host nodes.
   let hostSubtreeRoot = null;
 
@@ -2018,9 +2021,9 @@ function commitDeletionEffects(
 }
 
 function recursivelyTraverseDeletionEffects(
-  finishedRoot,
-  nearestMountedAncestor,
-  parent,
+  finishedRoot: FiberRoot,
+  nearestMountedAncestor: Fiber,
+  parent: Fiber,
 ) {
   // TODO: Use a static flag to skip trees that don't have unmount effects
   let child = parent.child;
@@ -2374,7 +2377,7 @@ function commitSuspenseHydrationCallbacks(
   }
 }
 
-function getRetryCache(finishedWork) {
+function getRetryCache(finishedWork: Fiber) {
   // TODO: Unify the interface for the retry cache so we don't have to switch
   // on the tag like this.
   switch (finishedWork.tag) {
@@ -4017,7 +4020,7 @@ function detachAlternateSiblings(parentFiber: Fiber) {
 
 function commitHookPassiveUnmountEffects(
   finishedWork: Fiber,
-  nearestMountedAncestor,
+  nearestMountedAncestor: null | Fiber,
   hookFlags: HookFlags,
 ) {
   if (shouldProfile(finishedWork)) {
diff --git a/packages/react-reconciler/src/ReactFiberHooks.js b/packages/react-reconciler/src/ReactFiberHooks.js
index eebd50915f31e..21454423a9a6e 100644
--- a/packages/react-reconciler/src/ReactFiberHooks.js
+++ b/packages/react-reconciler/src/ReactFiberHooks.js
@@ -1801,7 +1801,7 @@ function checkIfSnapshotChanged<T>(inst: StoreInstance<T>): boolean {
   }
 }
 
-function forceStoreRerender(fiber) {
+function forceStoreRerender(fiber: Fiber) {
   const root = enqueueConcurrentRenderForLane(fiber, SyncLane);
   if (root !== null) {
     scheduleUpdateOnFiber(root, fiber, SyncLane, NoTimestamp);
@@ -1934,7 +1934,7 @@ function mountRef<T>(initialValue: T): {current: T} {
           }
           return current;
         },
-        set current(value) {
+        set current(value: any) {
           if (currentlyRenderingFiber !== null && !didWarnAboutWrite) {
             if (hasBeenInitialized || !didCheckForLazyInit) {
               didWarnAboutWrite = true;
@@ -2736,7 +2736,7 @@ function entangleTransitionUpdate<S, A>(
   }
 }
 
-function markUpdateInDevTools<A>(fiber, lane, action: A): void {
+function markUpdateInDevTools<A>(fiber: Fiber, lane: Lane, action: A): void {
   if (__DEV__) {
     if (enableDebugTracing) {
       if (fiber.mode & DebugTracingMode) {
diff --git a/packages/react-reconciler/src/ReactFiberHydrationContext.js b/packages/react-reconciler/src/ReactFiberHydrationContext.js
index 4b5bc198a5e9f..ac7616920542a 100644
--- a/packages/react-reconciler/src/ReactFiberHydrationContext.js
+++ b/packages/react-reconciler/src/ReactFiberHydrationContext.js
@@ -338,7 +338,7 @@ function insertNonHydratedInstance(returnFiber: Fiber, fiber: Fiber) {
   warnNonhydratedInstance(returnFiber, fiber);
 }
 
-function tryHydrate(fiber, nextInstance) {
+function tryHydrate(fiber: Fiber, nextInstance: any) {
   switch (fiber.tag) {
     // HostSingleton is intentionally omitted. the hydration pathway for singletons is non-fallible
     // you can find it inlined in claimHydratableSingleton
diff --git a/packages/react-reconciler/src/ReactFiberReconciler.js b/packages/react-reconciler/src/ReactFiberReconciler.js
index 32ffad44a79e4..9d3861065a0ba 100644
--- a/packages/react-reconciler/src/ReactFiberReconciler.js
+++ b/packages/react-reconciler/src/ReactFiberReconciler.js
@@ -533,7 +533,7 @@ export function shouldError(fiber: Fiber): ?boolean {
   return shouldErrorImpl(fiber);
 }
 
-let shouldSuspendImpl = fiber => false;
+let shouldSuspendImpl = (fiber: Fiber) => false;
 
 export function shouldSuspend(fiber: Fiber): boolean {
   return shouldSuspendImpl(fiber);
diff --git a/packages/react-reconciler/src/ReactFiberRoot.js b/packages/react-reconciler/src/ReactFiberRoot.js
index 8ffec15b3a942..82f5655250084 100644
--- a/packages/react-reconciler/src/ReactFiberRoot.js
+++ b/packages/react-reconciler/src/ReactFiberRoot.js
@@ -45,11 +45,13 @@ export type RootState = {
 };
 
 function FiberRootNode(
-  containerInfo,
+  this: $FlowFixMe,
+  containerInfo: any,
+  // $FlowFixMe[missing-local-annot]
   tag,
-  hydrate,
-  identifierPrefix,
-  onRecoverableError,
+  hydrate: any,
+  identifierPrefix: any,
+  onRecoverableError: any,
 ) {
   this.tag = tag;
   this.containerInfo = containerInfo;
diff --git a/packages/react-reconciler/src/ReactFiberScope.js b/packages/react-reconciler/src/ReactFiberScope.js
index 3ce4ac784ce5b..9e92bd483e589 100644
--- a/packages/react-reconciler/src/ReactFiberScope.js
+++ b/packages/react-reconciler/src/ReactFiberScope.js
@@ -140,7 +140,10 @@ function collectNearestChildContextValues<T>(
   }
 }
 
-function DO_NOT_USE_queryAllNodes(fn: ReactScopeQuery): null | Array<Object> {
+function DO_NOT_USE_queryAllNodes(
+  this: $FlowFixMe,
+  fn: ReactScopeQuery,
+): null | Array<Object> {
   const currentFiber = getInstanceFromScope(this);
   if (currentFiber === null) {
     return null;
@@ -153,7 +156,10 @@ function DO_NOT_USE_queryAllNodes(fn: ReactScopeQuery): null | Array<Object> {
   return scopedNodes.length === 0 ? null : scopedNodes;
 }
 
-function DO_NOT_USE_queryFirstNode(fn: ReactScopeQuery): null | Object {
+function DO_NOT_USE_queryFirstNode(
+  this: $FlowFixMe,
+  fn: ReactScopeQuery,
+): null | Object {
   const currentFiber = getInstanceFromScope(this);
   if (currentFiber === null) {
     return null;
@@ -165,7 +171,7 @@ function DO_NOT_USE_queryFirstNode(fn: ReactScopeQuery): null | Object {
   return null;
 }
 
-function containsNode(node: Object): boolean {
+function containsNode(this: $FlowFixMe, node: Object): boolean {
   let fiber = getInstanceFromNode(node);
   while (fiber !== null) {
     if (fiber.tag === ScopeComponent && fiber.stateNode === this) {
@@ -176,7 +182,10 @@ function containsNode(node: Object): boolean {
   return false;
 }
 
-function getChildContextValues<T>(context: ReactContext<T>): Array<T> {
+function getChildContextValues<T>(
+  this: $FlowFixMe,
+  context: ReactContext<T>,
+): Array<T> {
   const currentFiber = getInstanceFromScope(this);
   if (currentFiber === null) {
     return [];
diff --git a/packages/react-reconciler/src/ReactFiberThrow.js b/packages/react-reconciler/src/ReactFiberThrow.js
index 7390a25e72d79..d02092b61dab3 100644
--- a/packages/react-reconciler/src/ReactFiberThrow.js
+++ b/packages/react-reconciler/src/ReactFiberThrow.js
@@ -123,6 +123,7 @@ function createClassErrorUpdate(
 
   const inst = fiber.stateNode;
   if (inst !== null && typeof inst.componentDidCatch === 'function') {
+    // $FlowFixMe[missing-this-annot]
     update.callback = function callback() {
       if (__DEV__) {
         markFailedErrorBoundaryForHotReloading(fiber);
diff --git a/packages/react-reconciler/src/ReactFiberTreeReflection.js b/packages/react-reconciler/src/ReactFiberTreeReflection.js
index ed10c62f99844..217a421248db4 100644
--- a/packages/react-reconciler/src/ReactFiberTreeReflection.js
+++ b/packages/react-reconciler/src/ReactFiberTreeReflection.js
@@ -117,7 +117,7 @@ export function isMounted(component: React$Component<any, any>): boolean {
   return getNearestMountedFiber(fiber) === fiber;
 }
 
-function assertIsMounted(fiber) {
+function assertIsMounted(fiber: Fiber) {
   if (getNearestMountedFiber(fiber) !== fiber) {
     throw new Error('Unable to find node on an unmounted component.');
   }
diff --git a/packages/react-reconciler/src/ReactFiberWorkLoop.js b/packages/react-reconciler/src/ReactFiberWorkLoop.js
index b6efb85e19728..cd9f1498c655b 100644
--- a/packages/react-reconciler/src/ReactFiberWorkLoop.js
+++ b/packages/react-reconciler/src/ReactFiberWorkLoop.js
@@ -1003,7 +1003,7 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
 
 // This is the entry point for every concurrent task, i.e. anything that
 // goes through Scheduler.
-function performConcurrentWorkOnRoot(root, didTimeout) {
+function performConcurrentWorkOnRoot(root: FiberRoot, didTimeout: boolean) {
   if (enableProfilerTimer && enableProfilerNestedUpdatePhase) {
     resetNestedUpdateFlag();
   }
@@ -1171,9 +1171,9 @@ function performConcurrentWorkOnRoot(root, didTimeout) {
 }
 
 function recoverFromConcurrentError(
-  root,
-  originallyAttemptedLanes,
-  errorRetryLanes,
+  root: FiberRoot,
+  originallyAttemptedLanes: Lanes,
+  errorRetryLanes: Lanes,
 ) {
   // If an error occurred during hydration, discard server response and fall
   // back to client side render.
@@ -1259,7 +1259,11 @@ export function queueRecoverableErrors(errors: Array<CapturedValue<mixed>>) {
   }
 }
 
-function finishConcurrentRender(root, exitStatus, lanes) {
+function finishConcurrentRender(
+  root: FiberRoot,
+  exitStatus: RootExitStatus,
+  lanes: Lanes,
+) {
   switch (exitStatus) {
     case RootInProgress:
     case RootFatalErrored: {
@@ -1439,7 +1443,7 @@ function isRenderConsistentWithExternalStores(finishedWork: Fiber): boolean {
   return true;
 }
 
-function markRootSuspended(root, suspendedLanes) {
+function markRootSuspended(root: FiberRoot, suspendedLanes: Lanes) {
   // When suspending, we should always exclude lanes that were pinged or (more
   // rarely, since we try to avoid it) updated during the render phase.
   // TODO: Lol maybe there's a better way to factor this besides this
@@ -1449,13 +1453,12 @@ function markRootSuspended(root, suspendedLanes) {
     suspendedLanes,
     workInProgressRootInterleavedUpdatedLanes,
   );
-  // $FlowFixMe[incompatible-call] found when upgrading Flow
   markRootSuspended_dontCallThisOneDirectly(root, suspendedLanes);
 }
 
 // This is the entry point for synchronous tasks that don't go
 // through Scheduler
-function performSyncWorkOnRoot(root) {
+function performSyncWorkOnRoot(root: FiberRoot) {
   if (enableProfilerTimer && enableProfilerNestedUpdatePhase) {
     syncNestedUpdateFlag();
   }
@@ -1764,7 +1767,7 @@ function resetSuspendedWorkLoopOnUnwind() {
   resetHooksOnUnwind();
 }
 
-function handleThrow(root, thrownValue): void {
+function handleThrow(root: FiberRoot, thrownValue: any): void {
   // A component threw an exception. Usually this is because it suspended, but
   // it also includes regular program errors.
   //
@@ -1902,7 +1905,7 @@ function shouldAttemptToSuspendUntilDataResolves() {
   return false;
 }
 
-function pushDispatcher(container) {
+function pushDispatcher(container: any) {
   prepareRendererToRender(container);
   const prevDispatcher = ReactCurrentDispatcher.current;
   ReactCurrentDispatcher.current = ContextOnlyDispatcher;
@@ -1916,7 +1919,7 @@ function pushDispatcher(container) {
   }
 }
 
-function popDispatcher(prevDispatcher) {
+function popDispatcher(prevDispatcher: any) {
   resetRendererAfterRender();
   ReactCurrentDispatcher.current = prevDispatcher;
 }
@@ -1931,7 +1934,7 @@ function pushCacheDispatcher() {
   }
 }
 
-function popCacheDispatcher(prevCacheDispatcher) {
+function popCacheDispatcher(prevCacheDispatcher: any) {
   if (enableCache) {
     ReactCurrentCache.current = prevCacheDispatcher;
   }
@@ -1973,6 +1976,7 @@ export function renderDidSuspendDelayIfPossible(): void {
     // pinged or updated while we were rendering.
     // TODO: Consider unwinding immediately, using the
     // SuspendedOnHydration mechanism.
+    // $FlowFixMe[incompatible-call] need null check workInProgressRoot
     markRootSuspended(workInProgressRoot, workInProgressRootRenderLanes);
   }
 }
@@ -3753,7 +3757,7 @@ export function warnAboutUpdateOnNotYetMountedFiberInDEV(fiber: Fiber) {
 let beginWork;
 if (__DEV__ && replayFailedUnitOfWorkWithInvokeGuardedCallback) {
   const dummyFiber = null;
-  beginWork = (current, unitOfWork, lanes) => {
+  beginWork = (current: null | Fiber, unitOfWork: Fiber, lanes: Lanes) => {
     // If a component throws an error, we replay it again in a synchronously
     // dispatched event, so that the debugger will treat it as an uncaught
     // error See ReactErrorUtils for more information.
@@ -3834,7 +3838,7 @@ if (__DEV__) {
   didWarnAboutUpdateInRenderForAnotherComponent = new Set();
 }
 
-function warnAboutRenderPhaseUpdatesInDEV(fiber) {
+function warnAboutRenderPhaseUpdatesInDEV(fiber: Fiber) {
   if (__DEV__) {
     if (ReactCurrentDebugFiberIsRenderingInDEV) {
       switch (fiber.tag) {
@@ -3893,7 +3897,8 @@ export function restorePendingUpdaters(root: FiberRoot, lanes: Lanes): void {
 }
 
 const fakeActCallbackNode = {};
-function scheduleCallback(priorityLevel, callback) {
+// $FlowFixMe[missing-local-annot]
+function scheduleCallback(priorityLevel: any, callback) {
   if (__DEV__) {
     // If we're currently inside an `act` scope, bypass Scheduler and push to
     // the `act` queue instead.
@@ -3910,7 +3915,7 @@ function scheduleCallback(priorityLevel, callback) {
   }
 }
 
-function cancelCallback(callbackNode) {
+function cancelCallback(callbackNode: any) {
   if (__DEV__ && callbackNode === fakeActCallbackNode) {
     return;
   }
diff --git a/packages/react-reconciler/src/ReactStrictModeWarnings.js b/packages/react-reconciler/src/ReactStrictModeWarnings.js
index a4a36a7aa3091..900fa68a6cd34 100644
--- a/packages/react-reconciler/src/ReactStrictModeWarnings.js
+++ b/packages/react-reconciler/src/ReactStrictModeWarnings.js
@@ -42,7 +42,7 @@ if (__DEV__) {
     return maybeStrictRoot;
   };
 
-  const setToSortedString = set => {
+  const setToSortedString = (set: Set<string>) => {
     const array = [];
     set.forEach(value => {
       array.push(value);
diff --git a/packages/react-refresh/src/ReactFreshRuntime.js b/packages/react-refresh/src/ReactFreshRuntime.js
index 6a936513a1e12..68f0e6281e4ce 100644
--- a/packages/react-refresh/src/ReactFreshRuntime.js
+++ b/packages/react-refresh/src/ReactFreshRuntime.js
@@ -123,7 +123,7 @@ function computeFullKey(signature: Signature): string {
   return fullKey;
 }
 
-function haveEqualSignatures(prevType, nextType) {
+function haveEqualSignatures(prevType: any, nextType: any) {
   const prevSignature = allSignaturesByType.get(prevType);
   const nextSignature = allSignaturesByType.get(nextType);
 
@@ -143,11 +143,11 @@ function haveEqualSignatures(prevType, nextType) {
   return true;
 }
 
-function isReactClass(type) {
+function isReactClass(type: any) {
   return type.prototype && type.prototype.isReactComponent;
 }
 
-function canPreserveStateBetween(prevType, nextType) {
+function canPreserveStateBetween(prevType: any, nextType: any) {
   if (isReactClass(prevType) || isReactClass(nextType)) {
     return false;
   }
@@ -157,7 +157,7 @@ function canPreserveStateBetween(prevType, nextType) {
   return false;
 }
 
-function resolveFamily(type) {
+function resolveFamily(type: any) {
   // Only check updated types to keep lookups fast.
   return updatedFamiliesByType.get(type);
 }
@@ -179,7 +179,7 @@ function cloneSet<T>(set: Set<T>): Set<T> {
 }
 
 // This is a safety mechanism to protect against rogue getters and Proxies.
-function getProperty(object, property) {
+function getProperty(object: any, property: string) {
   try {
     return object[property];
   } catch (err) {
@@ -489,7 +489,7 @@ export function injectIntoGlobalHook(globalObject: any): void {
 
     // Here, we just want to get a reference to scheduleRefresh.
     const oldInject = hook.inject;
-    hook.inject = function(injected) {
+    hook.inject = function(this: mixed, injected) {
       const id = oldInject.apply(this, arguments);
       if (
         typeof injected.scheduleRefresh === 'function' &&
@@ -518,6 +518,7 @@ export function injectIntoGlobalHook(globalObject: any): void {
     const oldOnCommitFiberRoot = hook.onCommitFiberRoot;
     const oldOnScheduleFiberRoot = hook.onScheduleFiberRoot || (() => {});
     hook.onScheduleFiberRoot = function(
+      this: mixed,
       id: number,
       root: FiberRoot,
       children: ReactNodeList,
@@ -533,6 +534,7 @@ export function injectIntoGlobalHook(globalObject: any): void {
       return oldOnScheduleFiberRoot.apply(this, arguments);
     };
     hook.onCommitFiberRoot = function(
+      this: mixed,
       id: number,
       root: FiberRoot,
       maybePriorityLevel: mixed,
diff --git a/packages/react-server-dom-relay/src/ReactFlightDOMRelayClientHostConfig.js b/packages/react-server-dom-relay/src/ReactFlightDOMRelayClientHostConfig.js
index a611259e76d07..a4dccf3513a35 100644
--- a/packages/react-server-dom-relay/src/ReactFlightDOMRelayClientHostConfig.js
+++ b/packages/react-server-dom-relay/src/ReactFlightDOMRelayClientHostConfig.js
@@ -44,6 +44,7 @@ export function resolveModuleReference<T>(
   return resolveModuleReferenceImpl(moduleData);
 }
 
+// $FlowFixMe[missing-local-annot]
 function parseModelRecursively(response: Response, parentObj, key, value) {
   if (typeof value === 'string') {
     return parseModelString(response, parentObj, key, value);
diff --git a/packages/react-server-dom-webpack/src/ReactFlightDOMClient.js b/packages/react-server-dom-webpack/src/ReactFlightDOMClient.js
index 13fbb0e32881d..1247113ef93d0 100644
--- a/packages/react-server-dom-webpack/src/ReactFlightDOMClient.js
+++ b/packages/react-server-dom-webpack/src/ReactFlightDOMClient.js
@@ -31,7 +31,7 @@ function startReadingFromStream(
   stream: ReadableStream,
 ): void {
   const reader = stream.getReader();
-  function progress({done, value}) {
+  function progress({done, value}: {done: boolean, value: ?any, ...}) {
     if (done) {
       close(response);
       return;
@@ -43,7 +43,7 @@ function startReadingFromStream(
       .then(progress)
       .catch(error);
   }
-  function error(e) {
+  function error(e: any) {
     reportGlobalError(response, e);
   }
   reader
diff --git a/packages/react-server-dom-webpack/src/ReactFlightDOMServerNode.js b/packages/react-server-dom-webpack/src/ReactFlightDOMServerNode.js
index 57dc2101ca8de..82ff956c99d72 100644
--- a/packages/react-server-dom-webpack/src/ReactFlightDOMServerNode.js
+++ b/packages/react-server-dom-webpack/src/ReactFlightDOMServerNode.js
@@ -7,7 +7,7 @@
  * @flow
  */
 
-import type {ReactModel} from 'react-server/src/ReactFlightServer';
+import type {Request, ReactModel} from 'react-server/src/ReactFlightServer';
 import type {Destination} from 'react-server/src/ReactServerStreamConfigNode';
 import type {BundlerConfig} from './ReactFlightServerWebpackBundlerConfig';
 import type {Writable} from 'stream';
@@ -20,7 +20,7 @@ import {
   abort,
 } from 'react-server/src/ReactFlightServer';
 
-function createDrainHandler(destination: Destination, request) {
+function createDrainHandler(destination: Destination, request: Request) {
   return () => startFlowing(request, destination);
 }
 
diff --git a/packages/react-server-dom-webpack/src/ReactFlightWebpackNodeLoader.js b/packages/react-server-dom-webpack/src/ReactFlightWebpackNodeLoader.js
index ff04bd97e48bb..0a9e2d59f691b 100644
--- a/packages/react-server-dom-webpack/src/ReactFlightWebpackNodeLoader.js
+++ b/packages/react-server-dom-webpack/src/ReactFlightWebpackNodeLoader.js
@@ -100,7 +100,7 @@ export async function getSource(
   return defaultGetSource(url, context, defaultGetSource);
 }
 
-function addExportNames(names, node) {
+function addExportNames(names: Array<string>, node: any) {
   switch (node.type) {
     case 'Identifier':
       names.push(node.name);
@@ -174,7 +174,7 @@ async function parseExportNamesInto(
   transformedSource: string,
   names: Array<string>,
   parentURL: string,
-  defaultTransformSource,
+  defaultTransformSource: TransformSourceFunction,
 ): Promise<void> {
   const {body} = acorn.parse(transformedSource, {
     ecmaVersion: '2019',
diff --git a/packages/react-server-dom-webpack/src/ReactFlightWebpackNodeRegister.js b/packages/react-server-dom-webpack/src/ReactFlightWebpackNodeRegister.js
index 3076a196badda..64157bd6eae9d 100644
--- a/packages/react-server-dom-webpack/src/ReactFlightWebpackNodeRegister.js
+++ b/packages/react-server-dom-webpack/src/ReactFlightWebpackNodeRegister.js
@@ -16,7 +16,11 @@ module.exports = function register() {
   const PROMISE_PROTOTYPE = Promise.prototype;
 
   const proxyHandlers = {
-    get: function(target, name, receiver) {
+    get: function(
+      target: {[string]: any},
+      name: string,
+      receiver: Proxy<{[string]: any}>,
+    ) {
       switch (name) {
         // These names are read by the Flight runtime if you end up using the exports object.
         case '$$typeof':
@@ -50,7 +54,8 @@ module.exports = function register() {
             // If this module is expected to return a Promise (such as an AsyncModule) then
             // we should resolve that with a client reference that unwraps the Promise on
             // the client.
-            const then = function then(resolve, reject) {
+            // $FlowFixMe[missing-local-annot]
+            const then = function then(resolve, reject: any) {
               const moduleReference: {[string]: any, ...} = {
                 $$typeof: MODULE_REFERENCE,
                 filepath: target.filepath,
@@ -82,7 +87,7 @@ module.exports = function register() {
       }
       return cachedReference;
     },
-    getPrototypeOf(target) {
+    getPrototypeOf(target: {[string]: any}) {
       // Pretend to be a Promise in case anyone asks.
       return PROMISE_PROTOTYPE;
     },
@@ -108,6 +113,7 @@ module.exports = function register() {
   const originalResolveFilename = Module._resolveFilename;
 
   // $FlowFixMe[prop-missing] found when upgrading Flow
+  // $FlowFixMe[missing-this-annot]
   Module._resolveFilename = function(request, parent, isMain, options) {
     const resolved = originalResolveFilename.apply(this, arguments);
     if (resolved.endsWith('.server.js')) {
diff --git a/packages/react-server-dom-webpack/src/ReactFlightWebpackPlugin.js b/packages/react-server-dom-webpack/src/ReactFlightWebpackPlugin.js
index 9182597c2b85a..0cdc501446822 100644
--- a/packages/react-server-dom-webpack/src/ReactFlightWebpackPlugin.js
+++ b/packages/react-server-dom-webpack/src/ReactFlightWebpackPlugin.js
@@ -144,6 +144,7 @@ export default class ReactFlightWebpackPlugin {
           new NullDependency.Template(),
         );
 
+        // $FlowFixMe[missing-local-annot]
         const handler = parser => {
           // We need to add all client references as dependency of something in the graph so
           // Webpack knows which entries need to know about the relevant chunks and include the
@@ -220,7 +221,8 @@ export default class ReactFlightWebpackPlugin {
               return c.id;
             });
 
-            function recordModule(id, module) {
+            // $FlowFixMe[missing-local-annot]
+            function recordModule(id: any, module) {
               // TODO: Hook into deps instead of the target module.
               // That way we know by the type of dep whether to include.
               // It also resolves conflicts when the same module is in multiple chunks.
diff --git a/packages/react-server-native-relay/src/ReactFlightNativeRelayClientHostConfig.js b/packages/react-server-native-relay/src/ReactFlightNativeRelayClientHostConfig.js
index 012d005ac83ad..ae2e2c0afb028 100644
--- a/packages/react-server-native-relay/src/ReactFlightNativeRelayClientHostConfig.js
+++ b/packages/react-server-native-relay/src/ReactFlightNativeRelayClientHostConfig.js
@@ -44,6 +44,7 @@ export function resolveModuleReference<T>(
   return resolveModuleReferenceImpl(moduleData);
 }
 
+// $FlowFixMe[missing-local-annot]
 function parseModelRecursively(response: Response, parentObj, key, value) {
   if (typeof value === 'string') {
     return parseModelString(response, parentObj, key, value);
diff --git a/packages/react-server/src/ReactFizzClassComponent.js b/packages/react-server/src/ReactFizzClassComponent.js
index 291a6d60f1cd2..f03117f65a440 100644
--- a/packages/react-server/src/ReactFizzClassComponent.js
+++ b/packages/react-server/src/ReactFizzClassComponent.js
@@ -60,7 +60,7 @@ if (__DEV__) {
     }
   };
 
-  warnOnUndefinedDerivedState = function(type, partialState) {
+  warnOnUndefinedDerivedState = function(type: any, partialState: any) {
     if (partialState === undefined) {
       const componentName = getComponentNameFromType(type) || 'Component';
       if (!didWarnAboutUndefinedDerivedState.has(componentName)) {
@@ -106,10 +106,11 @@ type InternalInstance = {
 };
 
 const classComponentUpdater = {
-  isMounted(inst) {
+  isMounted(inst: any) {
     return false;
   },
-  enqueueSetState(inst, payload, callback) {
+  // $FlowFixMe[missing-local-annot]
+  enqueueSetState(inst: any, payload: any, callback) {
     const internals: InternalInstance = getInstance(inst);
     if (internals.queue === null) {
       warnNoop(inst, 'setState');
@@ -122,7 +123,7 @@ const classComponentUpdater = {
       }
     }
   },
-  enqueueReplaceState(inst, payload, callback) {
+  enqueueReplaceState(inst: any, payload: any, callback: null) {
     const internals: InternalInstance = getInstance(inst);
     internals.replace = true;
     internals.queue = [payload];
@@ -132,7 +133,8 @@ const classComponentUpdater = {
       }
     }
   },
-  enqueueForceUpdate(inst, callback) {
+  // $FlowFixMe[missing-local-annot]
+  enqueueForceUpdate(inst: any, callback) {
     const internals: InternalInstance = getInstance(inst);
     if (internals.queue === null) {
       warnNoop(inst, 'forceUpdate');
@@ -532,7 +534,7 @@ function checkClassInstance(instance: any, ctor: any, newProps: any) {
   }
 }
 
-function callComponentWillMount(type, instance) {
+function callComponentWillMount(type: any, instance: any) {
   const oldState = instance.state;
 
   if (typeof instance.componentWillMount === 'function') {
diff --git a/packages/react-server/src/ReactFizzServer.js b/packages/react-server/src/ReactFizzServer.js
index 9922a36db8c43..bc7ece8424d12 100644
--- a/packages/react-server/src/ReactFizzServer.js
+++ b/packages/react-server/src/ReactFizzServer.js
@@ -719,7 +719,7 @@ function renderHostElement(
   popComponentStackInDEV(task);
 }
 
-function shouldConstruct(Component) {
+function shouldConstruct(Component: any) {
   return Component.prototype && Component.prototype.isReactComponent;
 }
 
@@ -1012,7 +1012,7 @@ function resolveDefaultProps(Component: any, baseProps: Object): Object {
 function renderForwardRef(
   request: Request,
   task: Task,
-  prevThenableState,
+  prevThenableState: null | ThenableState,
   type: any,
   props: Object,
   ref: any,
@@ -1301,6 +1301,7 @@ function renderElement(
   );
 }
 
+// $FlowFixMe[missing-local-annot]
 function validateIterable(iterable, iteratorFn: Function): void {
   if (__DEV__) {
     // We don't support rendering Generators because it's a mutation.
@@ -1505,7 +1506,11 @@ function renderNodeDestructiveImpl(
   }
 }
 
-function renderChildrenArray(request, task, children) {
+function renderChildrenArray(
+  request: Request,
+  task: Task,
+  children: Array<any>,
+) {
   const totalChildren = children.length;
   for (let i = 0; i < totalChildren; i++) {
     const prevTreeContext = task.treeContext;
@@ -2027,7 +2032,7 @@ function flushSubtree(
 
 function flushSegment(
   request: Request,
-  destination,
+  destination: Destination,
   segment: Segment,
 ): boolean {
   const boundary = segment.boundary;
diff --git a/packages/react-server/src/ReactFlightServer.js b/packages/react-server/src/ReactFlightServer.js
index 3a0876a56f5ec..f8a320a09903d 100644
--- a/packages/react-server/src/ReactFlightServer.js
+++ b/packages/react-server/src/ReactFlightServer.js
@@ -193,6 +193,7 @@ export function createRequest(
     identifierPrefix: identifierPrefix || '',
     identifierCount: 1,
     onError: onError === undefined ? defaultErrorHandler : onError,
+    // $FlowFixMe[missing-this-annot]
     toJSON: function(key: string, value: ReactModel): ReactJSONValue {
       return resolveModelToJSON(request, this, key, value);
     },
@@ -481,7 +482,7 @@ function escapeStringValue(value: string): string {
   }
 }
 
-function isObjectPrototype(object): boolean {
+function isObjectPrototype(object: any): boolean {
   if (!object) {
     return false;
   }
@@ -503,7 +504,7 @@ function isObjectPrototype(object): boolean {
   return true;
 }
 
-function isSimpleObject(object): boolean {
+function isSimpleObject(object: any): boolean {
   if (!isObjectPrototype(Object.getPrototypeOf(object))) {
     return false;
   }
@@ -529,7 +530,7 @@ function isSimpleObject(object): boolean {
   return true;
 }
 
-function objectName(object): string {
+function objectName(object: mixed): string {
   // $FlowFixMe[method-unbinding]
   const name = Object.prototype.toString.call(object);
   return name.replace(/^\[object (.*)\]$/, function(m, p0) {
diff --git a/packages/react-test-renderer/src/ReactTestRenderer.js b/packages/react-test-renderer/src/ReactTestRenderer.js
index 4645047977113..50153791e9fcc 100644
--- a/packages/react-test-renderer/src/ReactTestRenderer.js
+++ b/packages/react-test-renderer/src/ReactTestRenderer.js
@@ -133,6 +133,7 @@ function toJSON(inst: Instance | TextInstance): ReactTestRendererNode | null {
   }
 }
 
+// $FlowFixMe[missing-local-annot]
 function childrenToTree(node) {
   if (!node) {
     return null;
@@ -146,6 +147,7 @@ function childrenToTree(node) {
   return flatten(children.map(toTree));
 }
 
+// $FlowFixMe[missing-local-annot]
 function nodeAndSiblingsArray(nodeWithSibling) {
   const array = [];
   let node = nodeWithSibling;
@@ -156,6 +158,7 @@ function nodeAndSiblingsArray(nodeWithSibling) {
   return array;
 }
 
+// $FlowFixMe[missing-local-annot]
 function flatten(arr) {
   const result = [];
   const stack = [{i: 0, array: arr}];
@@ -449,6 +452,7 @@ function propsMatch(props: Object, filter: Object): boolean {
   return true;
 }
 
+// $FlowFixMe[missing-local-annot]
 function onRecoverableError(error) {
   // TODO: Expose onRecoverableError option to userspace
   // eslint-disable-next-line react-internal/no-production-logging, react-internal/warning-args
diff --git a/packages/react/src/ReactAct.js b/packages/react/src/ReactAct.js
index e6a1bd8e34492..a967562d088d7 100644
--- a/packages/react/src/ReactAct.js
+++ b/packages/react/src/ReactAct.js
@@ -205,7 +205,10 @@ export function act<T>(callback: () => T | Thenable<T>): Thenable<T> {
   }
 }
 
-function popActScope(prevActQueue, prevActScopeDepth) {
+function popActScope(
+  prevActQueue: null | Array<RendererTask>,
+  prevActScopeDepth: number,
+) {
   if (__DEV__) {
     if (prevActScopeDepth !== actScopeDepth - 1) {
       console.error(
@@ -252,7 +255,7 @@ function recursivelyFlushAsyncActWork<T>(
 }
 
 let isFlushing = false;
-function flushActQueue(queue) {
+function flushActQueue(queue: Array<RendererTask>) {
   if (__DEV__) {
     if (!isFlushing) {
       // Prevent re-entrance.
@@ -304,7 +307,7 @@ function flushActQueue(queue) {
 // environment it may cause the warning to fire too late.
 const queueSeveralMicrotasks =
   typeof queueMicrotask === 'function'
-    ? callback => {
+    ? (callback: () => void) => {
         queueMicrotask(() => queueMicrotask(callback));
       }
     : queueMacrotask;
diff --git a/packages/react/src/ReactChildren.js b/packages/react/src/ReactChildren.js
index 65c97c62cd7eb..6208186fbf15d 100644
--- a/packages/react/src/ReactChildren.js
+++ b/packages/react/src/ReactChildren.js
@@ -296,6 +296,7 @@ function forEachChildren(
 ): void {
   mapChildren(
     children,
+    // $FlowFixMe[missing-this-annot]
     function() {
       forEachFunc.apply(this, arguments);
       // Don't return anything.
diff --git a/packages/react/src/ReactContext.js b/packages/react/src/ReactContext.js
index 59bf7e11c7669..37ac671f69bed 100644
--- a/packages/react/src/ReactContext.js
+++ b/packages/react/src/ReactContext.js
@@ -9,6 +9,7 @@
 
 import {REACT_PROVIDER_TYPE, REACT_CONTEXT_TYPE} from 'shared/ReactSymbols';
 
+import type {ReactProviderType} from 'shared/ReactTypes';
 import type {ReactContext} from 'shared/ReactTypes';
 
 export function createContext<T>(defaultValue: T): ReactContext<T> {
@@ -66,7 +67,7 @@ export function createContext<T>(defaultValue: T): ReactContext<T> {
           }
           return context.Provider;
         },
-        set(_Provider) {
+        set(_Provider: ReactProviderType<T>) {
           context.Provider = _Provider;
         },
       },
@@ -74,7 +75,7 @@ export function createContext<T>(defaultValue: T): ReactContext<T> {
         get() {
           return context._currentValue;
         },
-        set(_currentValue) {
+        set(_currentValue: T) {
           context._currentValue = _currentValue;
         },
       },
@@ -82,7 +83,7 @@ export function createContext<T>(defaultValue: T): ReactContext<T> {
         get() {
           return context._currentValue2;
         },
-        set(_currentValue2) {
+        set(_currentValue2: T) {
           context._currentValue2 = _currentValue2;
         },
       },
@@ -90,7 +91,7 @@ export function createContext<T>(defaultValue: T): ReactContext<T> {
         get() {
           return context._threadCount;
         },
-        set(_threadCount) {
+        set(_threadCount: number) {
           context._threadCount = _threadCount;
         },
       },
@@ -110,7 +111,7 @@ export function createContext<T>(defaultValue: T): ReactContext<T> {
         get() {
           return context.displayName;
         },
-        set(displayName) {
+        set(displayName: void | string) {
           if (!hasWarnedAboutDisplayNameOnConsumer) {
             console.warn(
               'Setting `displayName` on Context.Consumer has no effect. ' +
diff --git a/packages/react/src/ReactLazy.js b/packages/react/src/ReactLazy.js
index 6537ef601e71b..04c5654bc80cf 100644
--- a/packages/react/src/ReactLazy.js
+++ b/packages/react/src/ReactLazy.js
@@ -144,6 +144,7 @@ export function lazy<T>(
         get() {
           return defaultProps;
         },
+        // $FlowFixMe[missing-local-annot]
         set(newDefaultProps) {
           console.error(
             'React.lazy(...): It is not supported to assign `defaultProps` to ' +
@@ -163,6 +164,7 @@ export function lazy<T>(
         get() {
           return propTypes;
         },
+        // $FlowFixMe[missing-local-annot]
         set(newPropTypes) {
           console.error(
             'React.lazy(...): It is not supported to assign `propTypes` to ' +
diff --git a/packages/scheduler/src/forks/Scheduler.js b/packages/scheduler/src/forks/Scheduler.js
index d43f457c2d680..236b3c6362830 100644
--- a/packages/scheduler/src/forks/Scheduler.js
+++ b/packages/scheduler/src/forks/Scheduler.js
@@ -329,6 +329,7 @@ function unstable_next<T>(eventHandler: () => T): T {
 function unstable_wrapCallback<T: (...Array<mixed>) => mixed>(callback: T): T {
   var parentPriorityLevel = currentPriorityLevel;
   // $FlowFixMe[incompatible-return]
+  // $FlowFixMe[missing-this-annot]
   return function() {
     // This is a fork of runWithPriority, inlined for performance.
     var previousPriorityLevel = currentPriorityLevel;
@@ -629,7 +630,9 @@ if (typeof localSetImmediate === 'function') {
   };
 }
 
-function requestHostCallback(callback) {
+function requestHostCallback(
+  callback: (hasTimeRemaining: boolean, initialTime: number) => boolean,
+) {
   scheduledHostCallback = callback;
   if (!isMessageLoopRunning) {
     isMessageLoopRunning = true;
@@ -637,7 +640,10 @@ function requestHostCallback(callback) {
   }
 }
 
-function requestHostTimeout(callback, ms: number) {
+function requestHostTimeout(
+  callback: (currentTime: number) => void,
+  ms: number,
+) {
   // $FlowFixMe[not-a-function] nullable value
   taskTimeoutID = localSetTimeout(() => {
     callback(getCurrentTime());
diff --git a/packages/scheduler/src/forks/SchedulerMock.js b/packages/scheduler/src/forks/SchedulerMock.js
index e276aab0ed6b8..6af3429b7fc25 100644
--- a/packages/scheduler/src/forks/SchedulerMock.js
+++ b/packages/scheduler/src/forks/SchedulerMock.js
@@ -322,6 +322,7 @@ function unstable_next<T>(eventHandler: () => T): T {
 function unstable_wrapCallback<T: (...Array<mixed>) => mixed>(callback: T): T {
   var parentPriorityLevel = currentPriorityLevel;
   // $FlowFixMe[incompatible-return]
+  // $FlowFixMe[missing-this-annot]
   return function() {
     // This is a fork of runWithPriority, inlined for performance.
     var previousPriorityLevel = currentPriorityLevel;
diff --git a/packages/scheduler/src/forks/SchedulerPostTask.js b/packages/scheduler/src/forks/SchedulerPostTask.js
index f78996f8028c4..43dd6a5dcdd74 100644
--- a/packages/scheduler/src/forks/SchedulerPostTask.js
+++ b/packages/scheduler/src/forks/SchedulerPostTask.js
@@ -168,7 +168,7 @@ function runTask<T>(
   }
 }
 
-function handleAbortError(error) {
+function handleAbortError(error: any) {
   // Abort errors are an implementation detail. We don't expose the
   // TaskController to the user, nor do we expose the promise that is returned
   // from `postTask`. So we should suppress them, since there's no way for the
diff --git a/packages/shared/ReactErrorUtils.js b/packages/shared/ReactErrorUtils.js
index b29110220736a..b2d9cde7b7124 100644
--- a/packages/shared/ReactErrorUtils.js
+++ b/packages/shared/ReactErrorUtils.js
@@ -72,6 +72,7 @@ export function invokeGuardedCallbackAndCatchFirstError<
   F,
   Context,
 >(
+  this: mixed,
   name: string | null,
   func: (a: A, b: B, c: C, d: D, e: E, f: F) => void,
   context: Context,
diff --git a/packages/shared/checkPropTypes.js b/packages/shared/checkPropTypes.js
index ba7b40f3c5f90..29d07f7afed18 100644
--- a/packages/shared/checkPropTypes.js
+++ b/packages/shared/checkPropTypes.js
@@ -16,7 +16,7 @@ import hasOwnProperty from 'shared/hasOwnProperty';
 
 const ReactDebugCurrentFrame = ReactSharedInternals.ReactDebugCurrentFrame;
 
-function setCurrentlyValidatingElement(element) {
+function setCurrentlyValidatingElement(element: any) {
   if (__DEV__) {
     if (element) {
       const owner = element._owner;
diff --git a/packages/shared/invokeGuardedCallbackImpl.js b/packages/shared/invokeGuardedCallbackImpl.js
index 017cc01046b37..07527be44a727 100644
--- a/packages/shared/invokeGuardedCallbackImpl.js
+++ b/packages/shared/invokeGuardedCallbackImpl.js
@@ -7,6 +7,7 @@
  * @flow
  */
 
+// $FlowFixMe[missing-this-annot]
 function invokeGuardedCallbackProd<Args: Array<mixed>, Context>(
   name: string | null,
   func: (...Args) => mixed,
@@ -62,6 +63,7 @@ if (__DEV__) {
     invokeGuardedCallbackImpl = function invokeGuardedCallbackDev<
       Args: Array<mixed>,
       Context,
+      // $FlowFixMe[missing-this-annot]
     >(name: string | null, func: (...Args) => mixed, context: Context): void {
       // If document doesn't exist we know for sure we will crash in this method
       // when we call document.createEvent(). However this can cause confusing
@@ -150,6 +152,7 @@ if (__DEV__) {
       let didSetError = false;
       let isCrossOriginError = false;
 
+      // $FlowFixMe[missing-local-annot]
       function handleWindowError(event) {
         error = event.error;
         didSetError = true;
diff --git a/packages/use-sync-external-store/src/useSyncExternalStoreWithSelector.js b/packages/use-sync-external-store/src/useSyncExternalStoreWithSelector.js
index 8d8f61e76bf7f..57935329f1e6f 100644
--- a/packages/use-sync-external-store/src/useSyncExternalStoreWithSelector.js
+++ b/packages/use-sync-external-store/src/useSyncExternalStoreWithSelector.js
@@ -44,7 +44,7 @@ export function useSyncExternalStoreWithSelector<Snapshot, Selection>(
     let hasMemo = false;
     let memoizedSnapshot;
     let memoizedSelection: Selection;
-    const memoizedSelector = nextSnapshot => {
+    const memoizedSelector = (nextSnapshot: Snapshot) => {
       if (!hasMemo) {
         // The first time the hook is called, there is no memoized result.
         hasMemo = true;
diff --git a/scripts/flow/config/flowconfig b/scripts/flow/config/flowconfig
index f746dfa43bdae..a4c50ad930eb6 100644
--- a/scripts/flow/config/flowconfig
+++ b/scripts/flow/config/flowconfig
@@ -43,7 +43,7 @@ untyped-type-import=error
 %CI_MAX_WORKERS%
 exact_by_default=true
 munge_underscores=false
-enforce_local_inference_annotations=false
+enforce_local_inference_annotations=true
 
 # Substituted by createFlowConfig.js:
 %REACT_RENDERER_FLOW_OPTIONS%
diff --git a/scripts/flow/createFlowConfigs.js b/scripts/flow/createFlowConfigs.js
index f9ce4a16a06bb..4a657ee2e6568 100644
--- a/scripts/flow/createFlowConfigs.js
+++ b/scripts/flow/createFlowConfigs.js
@@ -3,8 +3,6 @@
  *
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
- *
- * @flow
  */
 
 'use strict';