diff --git a/packages/react-client/src/ReactFlightClient.js b/packages/react-client/src/ReactFlightClient.js
index 3c9d26338b660..70e9f5ddaab95 100644
--- a/packages/react-client/src/ReactFlightClient.js
+++ b/packages/react-client/src/ReactFlightClient.js
@@ -393,7 +393,9 @@ function createElement(
     // We don't really need to add any of these but keeping them for good measure.
     // Unfortunately, _store is enumerable in jest matchers so for equality to
     // work, I need to keep it or make _store non-enumerable in the other file.
-    element._store = {};
+    element._store = ({}: {
+      validated?: boolean,
+    });
     Object.defineProperty(element._store, 'validated', {
       configurable: false,
       enumerable: false,
diff --git a/packages/react-debug-tools/src/ReactDebugHooks.js b/packages/react-debug-tools/src/ReactDebugHooks.js
index fbe4dc2f360a4..56c6e376dabb6 100644
--- a/packages/react-debug-tools/src/ReactDebugHooks.js
+++ b/packages/react-debug-tools/src/ReactDebugHooks.js
@@ -524,7 +524,7 @@ function buildTree(
   readHookLog: Array<HookLogEntry>,
   includeHooksSource: boolean,
 ): HooksTree {
-  const rootChildren = [];
+  const rootChildren: Array<HooksNode> = [];
   let prevStack = null;
   let levelChildren = rootChildren;
   let nativeHookID = 0;
@@ -557,7 +557,7 @@ function buildTree(
       // The remaining part of the new stack are custom hooks. Push them
       // to the tree.
       for (let j = stack.length - commonSteps - 1; j >= 1; j--) {
-        const children = [];
+        const children: Array<HooksNode> = [];
         const stackFrame = stack[j];
         const levelChild: HooksNode = {
           id: null,
diff --git a/packages/react-devtools-shared/src/backend/NativeStyleEditor/setupNativeStyleEditor.js b/packages/react-devtools-shared/src/backend/NativeStyleEditor/setupNativeStyleEditor.js
index ea9b8662f1be2..665a65f292d80 100644
--- a/packages/react-devtools-shared/src/backend/NativeStyleEditor/setupNativeStyleEditor.js
+++ b/packages/react-devtools-shared/src/backend/NativeStyleEditor/setupNativeStyleEditor.js
@@ -170,7 +170,7 @@ function measureStyle(
 }
 
 function shallowClone(object: Object): Object {
-  const cloned = {};
+  const cloned: {[string]: $FlowFixMe} = {};
   for (const n in object) {
     cloned[n] = object[n];
   }
diff --git a/packages/react-devtools-shared/src/backend/console.js b/packages/react-devtools-shared/src/backend/console.js
index 61ead6d7c478d..ac3d33306ecb4 100644
--- a/packages/react-devtools-shared/src/backend/console.js
+++ b/packages/react-devtools-shared/src/backend/console.js
@@ -79,7 +79,7 @@ const injectedRenderers: Map<
 > = new Map();
 
 let targetConsole: Object = console;
-let targetConsoleMethods = {};
+let targetConsoleMethods: {[string]: $FlowFixMe} = {};
 for (const method in console) {
   targetConsoleMethods[method] = console[method];
 }
@@ -97,7 +97,7 @@ export function dangerous_setTargetConsoleForTesting(
 ): void {
   targetConsole = targetConsoleForTesting;
 
-  targetConsoleMethods = {};
+  targetConsoleMethods = ({}: {[string]: $FlowFixMe});
   for (const method in targetConsole) {
     targetConsoleMethods[method] = console[method];
   }
@@ -179,7 +179,7 @@ export function patch({
       return;
     }
 
-    const originalConsoleMethods = {};
+    const originalConsoleMethods: {[string]: $FlowFixMe} = {};
 
     unpatchFn = () => {
       for (const method in originalConsoleMethods) {
@@ -318,7 +318,7 @@ export function patchForStrictMode() {
       return;
     }
 
-    const originalConsoleMethods = {};
+    const originalConsoleMethods: {[string]: $FlowFixMe} = {};
 
     unpatchForStrictModeFn = () => {
       for (const method in originalConsoleMethods) {
diff --git a/packages/react-devtools-shared/src/backend/legacy/renderer.js b/packages/react-devtools-shared/src/backend/legacy/renderer.js
index 1b667cf122185..69a193ecb08fc 100644
--- a/packages/react-devtools-shared/src/backend/legacy/renderer.js
+++ b/packages/react-devtools-shared/src/backend/legacy/renderer.js
@@ -785,7 +785,7 @@ export function attach(
 
       let owner = element._owner;
       if (owner) {
-        owners = [];
+        owners = ([]: Array<SerializedElement>);
         while (owner != null) {
           owners.push({
             displayName: getData(owner).displayName || 'Unknown',
@@ -807,8 +807,8 @@ export function attach(
     }
 
     // Not implemented
-    const errors = [];
-    const warnings = [];
+    const errors: Array<[string, number]> = [];
+    const warnings: Array<[string, number]> = [];
 
     return {
       id,
diff --git a/packages/react-devtools-shared/src/backend/legacy/utils.js b/packages/react-devtools-shared/src/backend/legacy/utils.js
index bc443c1dd164e..91dc1ff3798fe 100644
--- a/packages/react-devtools-shared/src/backend/legacy/utils.js
+++ b/packages/react-devtools-shared/src/backend/legacy/utils.js
@@ -22,7 +22,7 @@ export function decorateMany(
   source: Object,
   fns: {[attr: string]: Function, ...},
 ): Object {
-  const olds = {};
+  const olds: {[string]: $FlowFixMe} = {};
   for (const name in fns) {
     olds[name] = decorate(source, name, fns[name]);
   }
diff --git a/packages/react-devtools-shared/src/backend/profilingHooks.js b/packages/react-devtools-shared/src/backend/profilingHooks.js
index 47d173c55c3c4..7f7c766fc45e8 100644
--- a/packages/react-devtools-shared/src/backend/profilingHooks.js
+++ b/packages/react-devtools-shared/src/backend/profilingHooks.js
@@ -52,7 +52,7 @@ let supportsUserTiming =
 let supportsUserTimingV3 = false;
 if (supportsUserTiming) {
   const CHECK_V3_MARK = '__v3';
-  const markOptions = {};
+  const markOptions = ({}: {startTime?: number});
   Object.defineProperty(markOptions, 'startTime', {
     get: function() {
       supportsUserTimingV3 = true;
diff --git a/packages/react-devtools-shared/src/backend/renderer.js b/packages/react-devtools-shared/src/backend/renderer.js
index 54f95a2fbea6a..d05439e1360af 100644
--- a/packages/react-devtools-shared/src/backend/renderer.js
+++ b/packages/react-devtools-shared/src/backend/renderer.js
@@ -3252,7 +3252,7 @@ export function attach(
 
     let owners = null;
     if (_debugOwner) {
-      owners = [];
+      owners = ([]: Array<SerializedElement>);
       let owner: null | Fiber = _debugOwner;
       while (owner !== null) {
         owners.push(fiberToSerializedElement(owner));
@@ -3265,7 +3265,7 @@ export function attach(
 
     let hooks = null;
     if (usesHooks) {
-      const originalConsoleMethods = {};
+      const originalConsoleMethods: {[string]: $FlowFixMe} = {};
 
       // Temporarily disable all console logging before re-running the hook.
       for (const method in console) {
diff --git a/packages/react-devtools-shared/src/backend/utils.js b/packages/react-devtools-shared/src/backend/utils.js
index 5c0a9d7800bc6..03e7800819af6 100644
--- a/packages/react-devtools-shared/src/backend/utils.js
+++ b/packages/react-devtools-shared/src/backend/utils.js
@@ -20,8 +20,8 @@ export function cleanForBridge(
   path?: Array<string | number> = [],
 ): DehydratedData | null {
   if (data !== null) {
-    const cleanedPaths = [];
-    const unserializablePaths = [];
+    const cleanedPaths: Array<Array<string | number>> = [];
+    const unserializablePaths: Array<Array<string | number>> = [];
     const cleanedData = dehydrate(
       data,
       cleanedPaths,
diff --git a/packages/react-devtools-shared/src/devtools/utils.js b/packages/react-devtools-shared/src/devtools/utils.js
index 2e16d23c989bc..304483fd03f7f 100644
--- a/packages/react-devtools-shared/src/devtools/utils.js
+++ b/packages/react-devtools-shared/src/devtools/utils.js
@@ -198,7 +198,7 @@ const STACK_SOURCE_LOCATION = /([^\s]+) \((.+):(.+):(.+)\)/;
 export function stackToComponentSources(
   stack: string,
 ): Array<[string, ?Stack]> {
-  const out = [];
+  const out: Array<[string, ?Stack]> = [];
   stack
     .split(STACK_DELIMETER)
     .slice(1)
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/types.js b/packages/react-devtools-shared/src/devtools/views/Components/types.js
index 7e636e65dc52e..2eb4fc232cef2 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/types.js
+++ b/packages/react-devtools-shared/src/devtools/views/Components/types.js
@@ -125,14 +125,16 @@ export type InspectedElement = {
 
 // TODO: Add profiling type
 
+type Data =
+  | string
+  | Dehydrated
+  | Unserializable
+  | Array<Dehydrated>
+  | Array<Unserializable>
+  | {[string]: Data};
+
 export type DehydratedData = {
   cleaned: Array<Array<string | number>>,
-  data:
-    | string
-    | Dehydrated
-    | Unserializable
-    | Array<Dehydrated>
-    | Array<Unserializable>
-    | {[key: string]: string | Dehydrated | Unserializable, ...},
+  data: Data,
   unserializable: Array<Array<string | number>>,
 };
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 5d2e6e9cf5bb1..100815a5ad566 100644
--- a/packages/react-devtools-shared/src/devtools/views/Profiler/CommitTreeBuilder.js
+++ b/packages/react-devtools-shared/src/devtools/views/Profiler/CommitTreeBuilder.js
@@ -150,6 +150,7 @@ function updateTree(
 
   // Clone nodes before mutating them so edits don't affect them.
   const getClonedNode = (id: number): CommitTreeNode => {
+    // $FlowFixMe[prop-missing] - recommended fix is to use object spread operator
     const clonedNode = ((Object.assign(
       {},
       nodes.get(id),
diff --git a/packages/react-devtools-shared/src/devtools/views/utils.js b/packages/react-devtools-shared/src/devtools/views/utils.js
index 9f7aeccea32d4..d75c21bf802b7 100644
--- a/packages/react-devtools-shared/src/devtools/views/utils.js
+++ b/packages/react-devtools-shared/src/devtools/views/utils.js
@@ -134,7 +134,7 @@ export function serializeDataForCopy(props: Object): string {
 
 export function serializeHooksForCopy(hooks: HooksTree | null): string {
   // $FlowFixMe "HooksTree is not an object"
-  const cloned = Object.assign([], hooks);
+  const cloned = Object.assign(([]: Array<any>), hooks);
 
   const queue = [...cloned];
 
diff --git a/packages/react-devtools-shared/src/hook.js b/packages/react-devtools-shared/src/hook.js
index acecd13f06a69..cd135fa27fb34 100644
--- a/packages/react-devtools-shared/src/hook.js
+++ b/packages/react-devtools-shared/src/hook.js
@@ -29,7 +29,7 @@ export function installHook(target: any): DevToolsHook | null {
   }
 
   let targetConsole: Object = console;
-  let targetConsoleMethods = {};
+  let targetConsoleMethods: {[string]: $FlowFixMe} = {};
   for (const method in console) {
     targetConsoleMethods[method] = console[method];
   }
@@ -39,7 +39,7 @@ export function installHook(target: any): DevToolsHook | null {
   ): void {
     targetConsole = targetConsoleForTesting;
 
-    targetConsoleMethods = {};
+    targetConsoleMethods = ({}: {[string]: $FlowFixMe});
     for (const method in targetConsole) {
       targetConsoleMethods[method] = console[method];
     }
@@ -250,7 +250,7 @@ export function installHook(target: any): DevToolsHook | null {
       return;
     }
 
-    const originalConsoleMethods = {};
+    const originalConsoleMethods: {[string]: $FlowFixMe} = {};
 
     unpatchFn = () => {
       for (const method in originalConsoleMethods) {
@@ -516,9 +516,9 @@ export function installHook(target: any): DevToolsHook | null {
   }
 
   // TODO: More meaningful names for "rendererInterfaces" and "renderers".
-  const fiberRoots = {};
+  const fiberRoots: {[RendererID]: Set<mixed>} = {};
   const rendererInterfaces = new Map();
-  const listeners = {};
+  const listeners: {[string]: Array<Handler>} = {};
   const renderers = new Map();
 
   const hook: DevToolsHook = {
diff --git a/packages/react-devtools-shared/src/hooks/astUtils.js b/packages/react-devtools-shared/src/hooks/astUtils.js
index 24cd0e5ea4611..137e40b452730 100644
--- a/packages/react-devtools-shared/src/hooks/astUtils.js
+++ b/packages/react-devtools-shared/src/hooks/astUtils.js
@@ -362,7 +362,7 @@ function getPotentialHookDeclarationsFromAST(sourceAST: File): NodePath[] {
 export function getHookNamesMappingFromAST(
   sourceAST: File,
 ): $ReadOnlyArray<{name: string, start: Position}> {
-  const hookStack = [];
+  const hookStack: Array<{name: string, start: $FlowFixMe}> = [];
   const hookNames = [];
   const pushFrame = (name: string, node: Node) => {
     const nameInfo = {name, start: {...node.loc.start}};
diff --git a/packages/react-devtools-shared/src/hooks/generateHookMap.js b/packages/react-devtools-shared/src/hooks/generateHookMap.js
index 8d88ecc4c682e..be81581c92745 100644
--- a/packages/react-devtools-shared/src/hooks/generateHookMap.js
+++ b/packages/react-devtools-shared/src/hooks/generateHookMap.js
@@ -63,7 +63,7 @@ export function generateHookMap(sourceAST: File): HookMap {
   const hookNamesMapping = getHookNamesMappingFromAST(sourceAST);
   const namesMap: Map<string, number> = new Map();
   const names = [];
-  const mappings = [];
+  const mappings: Array<HookMapLine> = [];
 
   let currentLine = null;
   hookNamesMapping.forEach(({name, start}) => {
diff --git a/packages/react-devtools-shared/src/hydration.js b/packages/react-devtools-shared/src/hydration.js
index 1d046de69ca2f..e3dafa47b5d9c 100644
--- a/packages/react-devtools-shared/src/hydration.js
+++ b/packages/react-devtools-shared/src/hydration.js
@@ -16,7 +16,7 @@ import {
   setInObject,
 } from './utils';
 
-import type {DehydratedData} from 'react-devtools-shared/src/devtools/views/Components/types';
+import type {DehydratedData} from './devtools/views/Components/types';
 
 export const meta = {
   inspectable: (Symbol('inspectable'): symbol),
@@ -124,13 +124,7 @@ export function dehydrate(
   path: Array<string | number>,
   isPathAllowed: (path: Array<string | number>) => boolean,
   level?: number = 0,
-):
-  | string
-  | Dehydrated
-  | Unserializable
-  | Array<Dehydrated>
-  | Array<Unserializable>
-  | {[key: string]: string | Dehydrated | Unserializable, ...} {
+): $PropertyType<DehydratedData, 'data'> {
   const type = getDataType(data);
 
   let isPathAllowedCheck;
@@ -304,7 +298,9 @@ export function dehydrate(
       if (level >= LEVEL_THRESHOLD && !isPathAllowedCheck) {
         return createDehydrated(type, true, data, cleaned, path);
       } else {
-        const object = {};
+        const object: {
+          [string]: $PropertyType<DehydratedData, 'data'>,
+        } = {};
         getAllEnumerableKeys(data).forEach(key => {
           const name = key.toString();
           object[name] = dehydrate(
diff --git a/packages/react-devtools-shell/src/app/InspectableElements/CircularReferences.js b/packages/react-devtools-shell/src/app/InspectableElements/CircularReferences.js
index c1bc9dcf1e45d..f75b31005610c 100644
--- a/packages/react-devtools-shell/src/app/InspectableElements/CircularReferences.js
+++ b/packages/react-devtools-shell/src/app/InspectableElements/CircularReferences.js
@@ -14,8 +14,15 @@ const arrayTwo = [];
 arrayTwo.push(arrayOne);
 arrayOne.push(arrayTwo);
 
-const objectOne = {};
-const objectTwo = {objectOne};
+type ObjectOne = {
+  objectTwo?: ObjectTwo,
+};
+type ObjectTwo = {
+  objectOne: ObjectOne,
+};
+
+const objectOne: ObjectOne = {};
+const objectTwo: ObjectTwo = {objectOne};
 objectOne.objectTwo = objectTwo;
 
 export default function CircularReferences(): React.Node {
diff --git a/packages/react-devtools-shell/src/app/index.js b/packages/react-devtools-shell/src/app/index.js
index 6568fa7d3f019..f77d6c65d44a8 100644
--- a/packages/react-devtools-shell/src/app/index.js
+++ b/packages/react-devtools-shell/src/app/index.js
@@ -33,7 +33,7 @@ ignoreErrors([
 ignoreWarnings(['Warning: componentWillReceiveProps has been renamed']);
 ignoreLogs([]);
 
-const unmountFunctions = [];
+const unmountFunctions: Array<() => void | boolean> = [];
 
 function createContainer() {
   const container = document.createElement('div');
diff --git a/packages/react-devtools-timeline/src/TimelineSearchContext.js b/packages/react-devtools-timeline/src/TimelineSearchContext.js
index 830f98f1c35f7..604dbf180607a 100644
--- a/packages/react-devtools-timeline/src/TimelineSearchContext.js
+++ b/packages/react-devtools-timeline/src/TimelineSearchContext.js
@@ -40,7 +40,7 @@ type Action =
 
 type Dispatch = (action: Action) => void;
 
-const EMPTY_ARRAY = [];
+const EMPTY_ARRAY: Array<ReactComponentMeasure> = [];
 
 function reducer(state: State, action: Action): State {
   let {searchIndex, searchRegExp, searchResults, searchText} = state;
diff --git a/packages/react-dom-bindings/src/client/ReactDOMComponent.js b/packages/react-dom-bindings/src/client/ReactDOMComponent.js
index 13a41337f4c71..47047c98cecea 100644
--- a/packages/react-dom-bindings/src/client/ReactDOMComponent.js
+++ b/packages/react-dom-bindings/src/client/ReactDOMComponent.js
@@ -672,7 +672,7 @@ export function diffProperties(
       for (styleName in lastStyle) {
         if (lastStyle.hasOwnProperty(styleName)) {
           if (!styleUpdates) {
-            styleUpdates = {};
+            styleUpdates = ({}: {[string]: $FlowFixMe});
           }
           styleUpdates[styleName] = '';
         }
@@ -725,7 +725,7 @@ export function diffProperties(
             (!nextProp || !nextProp.hasOwnProperty(styleName))
           ) {
             if (!styleUpdates) {
-              styleUpdates = {};
+              styleUpdates = ({}: {[string]: string});
             }
             styleUpdates[styleName] = '';
           }
@@ -737,7 +737,7 @@ export function diffProperties(
             lastProp[styleName] !== nextProp[styleName]
           ) {
             if (!styleUpdates) {
-              styleUpdates = {};
+              styleUpdates = ({}: {[string]: $FlowFixMe});
             }
             styleUpdates[styleName] = nextProp[styleName];
           }
diff --git a/packages/react-dom-bindings/src/client/ReactDOMFloatClient.js b/packages/react-dom-bindings/src/client/ReactDOMFloatClient.js
index 14699940e31fd..cfb770711fb93 100644
--- a/packages/react-dom-bindings/src/client/ReactDOMFloatClient.js
+++ b/packages/react-dom-bindings/src/client/ReactDOMFloatClient.js
@@ -792,6 +792,7 @@ export function getResource(
 function preloadPropsFromRawProps(
   rawBorrowedProps: PreloadQualifyingProps,
 ): PreloadProps {
+  // $FlowFixMe[prop-missing] - recommended fix is to use object spread operator
   return Object.assign({}, rawBorrowedProps);
 }
 
@@ -805,6 +806,7 @@ function titlePropsFromRawProps(
 }
 
 function stylePropsFromRawProps(rawProps: StyleQualifyingProps): StyleProps {
+  // $FlowFixMe[prop-missing] - recommended fix is to use object spread operator
   const props: StyleProps = Object.assign({}, rawProps);
   props['data-precedence'] = rawProps.precedence;
   props.precedence = null;
@@ -813,6 +815,7 @@ function stylePropsFromRawProps(rawProps: StyleQualifyingProps): StyleProps {
 }
 
 function scriptPropsFromRawProps(rawProps: ScriptQualifyingProps): ScriptProps {
+  // $FlowFixMe[prop-missing] - recommended fix is to use object spread operator
   const props: ScriptProps = Object.assign({}, rawProps);
   return props;
 }
@@ -1318,7 +1321,9 @@ function acquireScriptResource(resource: ScriptResource): Instance {
 }
 
 function attachLoadListeners(instance: Instance, resource: StyleResource) {
-  const listeners = {};
+  const listeners: {
+    [string]: () => mixed,
+  } = {};
   listeners.load = onResourceLoad.bind(
     null,
     instance,
diff --git a/packages/react-dom-bindings/src/client/ReactDOMSelect.js b/packages/react-dom-bindings/src/client/ReactDOMSelect.js
index 3ecc9974c637d..ebf94c7b79fa3 100644
--- a/packages/react-dom-bindings/src/client/ReactDOMSelect.js
+++ b/packages/react-dom-bindings/src/client/ReactDOMSelect.js
@@ -77,7 +77,7 @@ function updateOptions(
 
   if (multiple) {
     const selectedValues = (propValue: Array<string>);
-    const selectedValue = {};
+    const selectedValue: {[string]: boolean} = {};
     for (let i = 0; i < selectedValues.length; i++) {
       // Prefix to avoid chaos with special keys.
       selectedValue['$' + selectedValues[i]] = true;
diff --git a/packages/react-dom-bindings/src/client/validateDOMNesting.js b/packages/react-dom-bindings/src/client/validateDOMNesting.js
index f3d016f46e476..c8960f817d57f 100644
--- a/packages/react-dom-bindings/src/client/validateDOMNesting.js
+++ b/packages/react-dom-bindings/src/client/validateDOMNesting.js
@@ -434,7 +434,7 @@ if (__DEV__) {
     return null;
   };
 
-  const didWarn = {};
+  const didWarn: {[string]: boolean} = {};
 
   validateDOMNesting = function(
     childTag: ?string,
diff --git a/packages/react-dom-bindings/src/events/checkPassiveEvents.js b/packages/react-dom-bindings/src/events/checkPassiveEvents.js
index 57267c47befaf..a0410117e7c8c 100644
--- a/packages/react-dom-bindings/src/events/checkPassiveEvents.js
+++ b/packages/react-dom-bindings/src/events/checkPassiveEvents.js
@@ -15,7 +15,9 @@ export let passiveBrowserEventsSupported: boolean = false;
 // https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/addEventListener#Safely_detecting_option_support
 if (canUseDOM) {
   try {
-    const options = {};
+    const options: {
+      passive?: void,
+    } = {};
     Object.defineProperty(options, 'passive', {
       get: function() {
         passiveBrowserEventsSupported = true;
diff --git a/packages/react-dom-bindings/src/server/ReactDOMServerFormatConfig.js b/packages/react-dom-bindings/src/server/ReactDOMServerFormatConfig.js
index 843c4bee1ff6d..921424e852e3f 100644
--- a/packages/react-dom-bindings/src/server/ReactDOMServerFormatConfig.js
+++ b/packages/react-dom-bindings/src/server/ReactDOMServerFormatConfig.js
@@ -191,7 +191,7 @@ export function createResponseState(
       : stringToPrecomputedChunk(
           '<script nonce="' + escapeTextForBrowser(nonce) + '">',
         );
-  const bootstrapChunks = [];
+  const bootstrapChunks: Array<Chunk | PrecomputedChunk> = [];
   let externalRuntimeDesc = null;
   let streamingFormat = ScriptStreamingFormat;
   if (bootstrapScriptContent !== undefined) {
@@ -2777,7 +2777,7 @@ export function writeInitialResources(
     }
   }
 
-  const target = [];
+  const target: Array<Chunk | PrecomputedChunk> = [];
 
   const {
     charset,
@@ -2902,7 +2902,7 @@ export function writeImmediateResources(
     }
   }
 
-  const target = [];
+  const target: Array<Chunk | PrecomputedChunk> = [];
 
   const {
     charset,
diff --git a/packages/react-dom-bindings/src/shared/DOMProperty.js b/packages/react-dom-bindings/src/shared/DOMProperty.js
index 7caa042296837..1c73df1a6c288 100644
--- a/packages/react-dom-bindings/src/shared/DOMProperty.js
+++ b/packages/react-dom-bindings/src/shared/DOMProperty.js
@@ -70,8 +70,8 @@ export const VALID_ATTRIBUTE_NAME_REGEX: RegExp = new RegExp(
   '^[' + ATTRIBUTE_NAME_START_CHAR + '][' + ATTRIBUTE_NAME_CHAR + ']*$',
 );
 
-const illegalAttributeNameCache = {};
-const validatedAttributeNameCache = {};
+const illegalAttributeNameCache: {[string]: boolean} = {};
+const validatedAttributeNameCache: {[string]: boolean} = {};
 
 export function isAttributeNameSafe(attributeName: string): boolean {
   if (hasOwnProperty.call(validatedAttributeNameCache, attributeName)) {
@@ -239,7 +239,7 @@ function PropertyInfoRecord(
 // When adding attributes to this list, be sure to also add them to
 // the `possibleStandardNames` module to ensure casing and incorrect
 // name warnings.
-const properties = {};
+const properties: {[string]: $FlowFixMe} = {};
 
 // These props are reserved by React. They shouldn't be written to the DOM.
 const reservedProps = [
diff --git a/packages/react-dom-bindings/src/shared/ReactDOMResourceValidation.js b/packages/react-dom-bindings/src/shared/ReactDOMResourceValidation.js
index dace0e1f96b47..d52656e5a3966 100644
--- a/packages/react-dom-bindings/src/shared/ReactDOMResourceValidation.js
+++ b/packages/react-dom-bindings/src/shared/ReactDOMResourceValidation.js
@@ -108,7 +108,7 @@ export function validatePreloadResourceDifference(
       let extraProps = null;
       let differentProps = null;
       if (originalProps.media != null && latestProps.media == null) {
-        missingProps = missingProps || {};
+        missingProps = missingProps || ({}: {[string]: $FlowFixMe});
         missingProps.media = originalProps.media;
       }
 
@@ -118,10 +118,12 @@ export function validatePreloadResourceDifference(
 
         if (propValue != null && propValue !== originalValue) {
           if (originalValue == null) {
-            extraProps = extraProps || {};
+            extraProps = extraProps || ({}: {[string]: $FlowFixMe});
             extraProps[propName] = propValue;
           } else {
-            differentProps = differentProps || {};
+            differentProps =
+              differentProps ||
+              ({}: {[string]: {latest: $FlowFixMe, original: $FlowFixMe}});
             differentProps[propName] = {
               original: originalValue,
               latest: propValue,
@@ -166,7 +168,7 @@ export function validateStyleResourceDifference(
     let extraProps = null;
     let differentProps = null;
     if (originalProps.media != null && latestProps.media == null) {
-      missingProps = missingProps || {};
+      missingProps = missingProps || ({}: {[string]: $FlowFixMe});
       missingProps.media = originalProps.media;
     }
 
@@ -177,10 +179,12 @@ export function validateStyleResourceDifference(
       if (propValue != null && propValue !== originalValue) {
         propName = propName === 'data-precedence' ? 'precedence' : propName;
         if (originalValue == null) {
-          extraProps = extraProps || {};
+          extraProps = extraProps || ({}: {[string]: $FlowFixMe});
           extraProps[propName] = propValue;
         } else {
-          differentProps = differentProps || {};
+          differentProps =
+            differentProps ||
+            ({}: {[string]: {latest: $FlowFixMe, original: $FlowFixMe}});
           differentProps[propName] = {
             original: originalValue,
             latest: propValue,
@@ -229,10 +233,12 @@ export function validateScriptResourceDifference(
 
       if (propValue != null && propValue !== originalValue) {
         if (originalValue == null) {
-          extraProps = extraProps || {};
+          extraProps = extraProps || ({}: {[string]: $FlowFixMe});
           extraProps[propName] = propValue;
         } else {
-          differentProps = differentProps || {};
+          differentProps =
+            differentProps ||
+            ({}: {[string]: {latest: $FlowFixMe, original: $FlowFixMe}});
           differentProps[propName] = {
             original: originalValue,
             latest: propValue,
@@ -304,13 +310,15 @@ export function validateStyleAndHintProps(
             !(preloadValue == null && styleValue == null)
           ) {
             if (styleValue == null) {
-              missingProps = missingProps || {};
+              missingProps = missingProps || ({}: {[string]: $FlowFixMe});
               missingProps[propName] = preloadValue;
             } else if (preloadValue == null) {
-              extraProps = extraProps || {};
+              extraProps = extraProps || ({}: {[string]: $FlowFixMe});
               extraProps[propName] = styleValue;
             } else {
-              differentProps = differentProps || {};
+              differentProps =
+                differentProps ||
+                ({}: {[string]: {latest: $FlowFixMe, original: $FlowFixMe}});
               differentProps[propName] = {
                 original: preloadValue,
                 latest: styleValue,
@@ -383,13 +391,15 @@ export function validateScriptAndHintProps(
             !(preloadValue == null && scriptValue == null)
           ) {
             if (scriptValue == null) {
-              missingProps = missingProps || {};
+              missingProps = missingProps || ({}: {[string]: $FlowFixMe});
               missingProps[propName] = preloadValue;
             } else if (preloadValue == null) {
-              extraProps = extraProps || {};
+              extraProps = extraProps || ({}: {[string]: $FlowFixMe});
               extraProps[propName] = scriptValue;
             } else {
-              differentProps = differentProps || {};
+              differentProps =
+                differentProps ||
+                ({}: {[string]: {latest: $FlowFixMe, original: $FlowFixMe}});
               differentProps[propName] = {
                 original: preloadValue,
                 latest: scriptValue,
@@ -419,8 +429,8 @@ function warnDifferentProps(
   urlPropKey: string,
   originalName: string,
   latestName: string,
-  extraProps: ?{[string]: any},
-  missingProps: ?{[string]: any},
+  extraProps: ?{[string]: $FlowFixMe},
+  missingProps: ?{[string]: $FlowFixMe},
   differentProps: ?{[string]: {original: any, latest: any}},
 ): void {
   if (__DEV__) {
@@ -510,13 +520,15 @@ export function validateURLKeyedUpdatedProps(
           !(pendingValue == null && currentValue == null)
         ) {
           if (pendingValue == null) {
-            missingProps = missingProps || {};
+            missingProps = missingProps || ({}: {[string]: mixed});
             missingProps[propName] = currentValue;
           } else if (currentValue == null) {
-            extraProps = extraProps || {};
+            extraProps = extraProps || ({}: {[string]: mixed});
             extraProps[propName] = pendingValue;
           } else {
-            differentProps = differentProps || {};
+            differentProps =
+              differentProps ||
+              ({}: {[string]: {latest: mixed, original: mixed}});
             differentProps[propName] = {
               original: currentValue,
               latest: pendingValue,
@@ -534,16 +546,19 @@ export function validateURLKeyedUpdatedProps(
         let comparisonStatement = '';
         if (missingProps !== null && typeof missingProps === 'object') {
           for (const propName in missingProps) {
+            // $FlowFixMe[incompatible-type]
             comparisonStatement += `\n  ${propName}: missing or null in latest props, "${missingProps[propName]}" in original props`;
           }
         }
         if (extraProps !== null && typeof extraProps === 'object') {
           for (const propName in extraProps) {
+            // $FlowFixMe[incompatible-type]
             comparisonStatement += `\n  ${propName}: "${extraProps[propName]}" in latest props, missing or null in original props`;
           }
         }
         if (differentProps !== null && typeof differentProps === 'object') {
           for (const propName in differentProps) {
+            // $FlowFixMe[incompatible-type]
             comparisonStatement += `\n  ${propName}: "${differentProps[propName].latest}" in latest props, "${differentProps[propName].original}" in original props`;
           }
         }
diff --git a/packages/react-native-renderer/src/ReactNativeAttributePayload.js b/packages/react-native-renderer/src/ReactNativeAttributePayload.js
index bda088c01f06b..ecfc19e359e7e 100644
--- a/packages/react-native-renderer/src/ReactNativeAttributePayload.js
+++ b/packages/react-native-renderer/src/ReactNativeAttributePayload.js
@@ -30,7 +30,7 @@ const emptyObject = {};
 type NestedNode = Array<NestedNode> | Object;
 
 // Tracks removed keys
-let removedKeys = null;
+let removedKeys: {[string]: boolean} | null = null;
 let removedKeyCount = 0;
 
 const deepDifferOptions = {
@@ -345,7 +345,9 @@ function diffProperties(
       // case: !Object is the default case
       if (defaultDiffer(prevProp, nextProp)) {
         // a normal leaf has changed
-        (updatePayload || (updatePayload = {}))[propKey] = nextProp;
+        (updatePayload || (updatePayload = ({}: {[string]: $FlowFixMe})))[
+          propKey
+        ] = nextProp;
       }
     } else if (
       typeof attributeConfig.diff === 'function' ||
@@ -363,7 +365,9 @@ function diffProperties(
             ? // $FlowFixMe[incompatible-use] found when upgrading Flow
               attributeConfig.process(nextProp)
             : nextProp;
-        (updatePayload || (updatePayload = {}))[propKey] = nextValue;
+        (updatePayload || (updatePayload = ({}: {[string]: $FlowFixMe})))[
+          propKey
+        ] = nextValue;
       }
     } else {
       // default: fallthrough case when nested properties are defined
@@ -417,9 +421,11 @@ function diffProperties(
     ) {
       // case: CustomAttributeConfiguration | !Object
       // Flag the leaf property for removal by sending a sentinel.
-      (updatePayload || (updatePayload = {}))[propKey] = null;
+      (updatePayload || (updatePayload = ({}: {[string]: $FlowFixMe})))[
+        propKey
+      ] = null;
       if (!removedKeys) {
-        removedKeys = {};
+        removedKeys = ({}: {[string]: boolean});
       }
       if (!removedKeys[propKey]) {
         removedKeys[propKey] = true;
diff --git a/packages/react-reconciler/src/ReactChildFiber.js b/packages/react-reconciler/src/ReactChildFiber.js
index 09f318969a001..6d9c6e83f9a6e 100644
--- a/packages/react-reconciler/src/ReactChildFiber.js
+++ b/packages/react-reconciler/src/ReactChildFiber.js
@@ -54,15 +54,15 @@ let warnForMissingKey = (child: mixed, returnFiber: Fiber) => {};
 if (__DEV__) {
   didWarnAboutMaps = false;
   didWarnAboutGenerators = false;
-  didWarnAboutStringRefs = {};
+  didWarnAboutStringRefs = ({}: {[string]: boolean});
 
   /**
    * Warn if there's no key explicitly set on dynamic arrays of children or
    * object keys are not valid. This allows us to keep track of children between
    * updates.
    */
-  ownerHasKeyUseWarning = {};
-  ownerHasFunctionTypeWarning = {};
+  ownerHasKeyUseWarning = ({}: {[string]: boolean});
+  ownerHasFunctionTypeWarning = ({}: {[string]: boolean});
 
   warnForMissingKey = (child: mixed, returnFiber: Fiber) => {
     if (child === null || typeof child !== 'object') {
diff --git a/packages/react-reconciler/src/ReactFiberBeginWork.js b/packages/react-reconciler/src/ReactFiberBeginWork.js
index cb48a75525349..01c531ab034e9 100644
--- a/packages/react-reconciler/src/ReactFiberBeginWork.js
+++ b/packages/react-reconciler/src/ReactFiberBeginWork.js
@@ -303,15 +303,15 @@ let didWarnAboutTailOptions;
 let didWarnAboutDefaultPropsOnFunctionComponent;
 
 if (__DEV__) {
-  didWarnAboutBadClass = {};
-  didWarnAboutModulePatternComponent = {};
-  didWarnAboutContextTypeOnFunctionComponent = {};
-  didWarnAboutGetDerivedStateOnFunctionComponent = {};
-  didWarnAboutFunctionRefs = {};
+  didWarnAboutBadClass = ({}: {[string]: boolean});
+  didWarnAboutModulePatternComponent = ({}: {[string]: boolean});
+  didWarnAboutContextTypeOnFunctionComponent = ({}: {[string]: boolean});
+  didWarnAboutGetDerivedStateOnFunctionComponent = ({}: {[string]: boolean});
+  didWarnAboutFunctionRefs = ({}: {[string]: boolean});
   didWarnAboutReassigningProps = false;
-  didWarnAboutRevealOrder = {};
-  didWarnAboutTailOptions = {};
-  didWarnAboutDefaultPropsOnFunctionComponent = {};
+  didWarnAboutRevealOrder = ({}: {[empty]: boolean});
+  didWarnAboutTailOptions = ({}: {[string]: boolean});
+  didWarnAboutDefaultPropsOnFunctionComponent = ({}: {[string]: boolean});
 }
 
 export function reconcileChildren(
diff --git a/packages/react-reconciler/src/ReactFiberClassComponent.js b/packages/react-reconciler/src/ReactFiberClassComponent.js
index 2443db472ba1d..9fd6d5dc3169d 100644
--- a/packages/react-reconciler/src/ReactFiberClassComponent.js
+++ b/packages/react-reconciler/src/ReactFiberClassComponent.js
@@ -77,7 +77,9 @@ import {
   setIsStrictModeForDevtools,
 } from './ReactFiberDevToolsHook';
 
-const fakeInternalInstance = {};
+const fakeInternalInstance: {
+  _processChildContext?: () => empty,
+} = {};
 
 let didWarnAboutStateAssignmentForComponent;
 let didWarnAboutUninitializedState;
@@ -139,7 +141,7 @@ if (__DEV__) {
   // exception.
   Object.defineProperty(fakeInternalInstance, '_processChildContext', {
     enumerable: false,
-    value: function() {
+    value: function(): empty {
       throw new Error(
         '_processChildContext is not available in React 16+. This likely ' +
           'means you have multiple copies of React and are attempting to nest ' +
diff --git a/packages/react-reconciler/src/ReactFiberContext.js b/packages/react-reconciler/src/ReactFiberContext.js
index 0f1a5219c09c3..264e39f05666b 100644
--- a/packages/react-reconciler/src/ReactFiberContext.js
+++ b/packages/react-reconciler/src/ReactFiberContext.js
@@ -21,7 +21,7 @@ import {createCursor, push, pop} from './ReactFiberStack';
 let warnedAboutMissingGetChildContext;
 
 if (__DEV__) {
-  warnedAboutMissingGetChildContext = {};
+  warnedAboutMissingGetChildContext = ({}: {[string]: boolean});
 }
 
 // $FlowFixMe[incompatible-exact]
@@ -98,7 +98,7 @@ function getMaskedContext(
       return instance.__reactInternalMemoizedMaskedChildContext;
     }
 
-    const context = {};
+    const context: {[string]: $FlowFixMe} = {};
     for (const key in contextTypes) {
       context[key] = unmaskedContext[key];
     }
diff --git a/packages/react-reconciler/src/ReactFiberHooks.js b/packages/react-reconciler/src/ReactFiberHooks.js
index 21454423a9a6e..53d726aeb2d81 100644
--- a/packages/react-reconciler/src/ReactFiberHooks.js
+++ b/packages/react-reconciler/src/ReactFiberHooks.js
@@ -144,6 +144,7 @@ import {
   createThenableState,
 } from './ReactFiberThenable';
 import type {ThenableState} from './ReactFiberThenable';
+import type {BatchConfigTransition} from './ReactFiberTracingMarkerComponent';
 
 const {ReactCurrentDispatcher, ReactCurrentBatchConfig} = ReactSharedInternals;
 
@@ -2383,7 +2384,7 @@ function startTransition(
   setPending(true);
 
   const prevTransition = ReactCurrentBatchConfig.transition;
-  ReactCurrentBatchConfig.transition = {};
+  ReactCurrentBatchConfig.transition = ({}: BatchConfigTransition);
   const currentTransition = ReactCurrentBatchConfig.transition;
 
   if (enableTransitionTracing) {
@@ -2408,6 +2409,7 @@ function startTransition(
     if (__DEV__) {
       if (prevTransition === null && currentTransition._updatedFibers) {
         const updatedFibersCount = currentTransition._updatedFibers.size;
+        currentTransition._updatedFibers.clear();
         if (updatedFibersCount > 10) {
           console.warn(
             'Detected a large number of updates inside startTransition. ' +
@@ -2415,7 +2417,6 @@ function startTransition(
               'Otherwise concurrent mode guarantees are off the table.',
           );
         }
-        currentTransition._updatedFibers.clear();
       }
     }
   }
diff --git a/packages/react-reconciler/src/ReactFiberReconciler.js b/packages/react-reconciler/src/ReactFiberReconciler.js
index 9d3861065a0ba..d6e2b1c85dc23 100644
--- a/packages/react-reconciler/src/ReactFiberReconciler.js
+++ b/packages/react-reconciler/src/ReactFiberReconciler.js
@@ -136,7 +136,7 @@ let didWarnAboutFindNodeInStrictMode;
 
 if (__DEV__) {
   didWarnAboutNestedUpdates = false;
-  didWarnAboutFindNodeInStrictMode = {};
+  didWarnAboutFindNodeInStrictMode = ({}: {[string]: boolean});
 }
 
 function getContextForSubtree(
diff --git a/packages/react-reconciler/src/ReactFiberScope.js b/packages/react-reconciler/src/ReactFiberScope.js
index 9e92bd483e589..c663f4a1c537f 100644
--- a/packages/react-reconciler/src/ReactFiberScope.js
+++ b/packages/react-reconciler/src/ReactFiberScope.js
@@ -149,7 +149,7 @@ function DO_NOT_USE_queryAllNodes(
     return null;
   }
   const child = currentFiber.child;
-  const scopedNodes = [];
+  const scopedNodes: Array<any> = [];
   if (child !== null) {
     collectScopedNodesFromChildren(child, fn, scopedNodes);
   }
@@ -191,7 +191,7 @@ function getChildContextValues<T>(
     return [];
   }
   const child = currentFiber.child;
-  const childContextValues = [];
+  const childContextValues: Array<T> = [];
   if (child !== null) {
     collectNearestChildContextValues(child, context, childContextValues);
   }
diff --git a/packages/react-server-dom-webpack/src/ReactFlightWebpackNodeLoader.js b/packages/react-server-dom-webpack/src/ReactFlightWebpackNodeLoader.js
index 0a9e2d59f691b..8f2f5539292aa 100644
--- a/packages/react-server-dom-webpack/src/ReactFlightWebpackNodeLoader.js
+++ b/packages/react-server-dom-webpack/src/ReactFlightWebpackNodeLoader.js
@@ -237,7 +237,7 @@ export async function transformSource(
       throw new Error('Expected source to have been transformed to a string.');
     }
 
-    const names = [];
+    const names: Array<string> = [];
     await parseExportNamesInto(
       transformedSource,
       names,
diff --git a/packages/react-server-dom-webpack/src/ReactFlightWebpackNodeRegister.js b/packages/react-server-dom-webpack/src/ReactFlightWebpackNodeRegister.js
index 64157bd6eae9d..13fb79d78b8ca 100644
--- a/packages/react-server-dom-webpack/src/ReactFlightWebpackNodeRegister.js
+++ b/packages/react-server-dom-webpack/src/ReactFlightWebpackNodeRegister.js
@@ -17,9 +17,9 @@ module.exports = function register() {
 
   const proxyHandlers = {
     get: function(
-      target: {[string]: any},
+      target: {[string]: $FlowFixMe},
       name: string,
-      receiver: Proxy<{[string]: any}>,
+      receiver: Proxy<{[string]: $FlowFixMe}>,
     ) {
       switch (name) {
         // These names are read by the Flight runtime if you end up using the exports object.
@@ -87,7 +87,7 @@ module.exports = function register() {
       }
       return cachedReference;
     },
-    getPrototypeOf(target: {[string]: any}) {
+    getPrototypeOf(target: {[string]: $FlowFixMe}) {
       // Pretend to be a Promise in case anyone asks.
       return PROMISE_PROTOTYPE;
     },
diff --git a/packages/react-server-dom-webpack/src/ReactFlightWebpackPlugin.js b/packages/react-server-dom-webpack/src/ReactFlightWebpackPlugin.js
index 0cdc501446822..5ce11be89e162 100644
--- a/packages/react-server-dom-webpack/src/ReactFlightWebpackPlugin.js
+++ b/packages/react-server-dom-webpack/src/ReactFlightWebpackPlugin.js
@@ -215,14 +215,18 @@ export default class ReactFlightWebpackPlugin {
             return;
           }
 
-          const json = {};
+          const json: {
+            [string]: {
+              [string]: {chunks: $FlowFixMe, id: $FlowFixMe, name: string},
+            },
+          } = {};
           compilation.chunkGroups.forEach(function(chunkGroup) {
             const chunkIds = chunkGroup.chunks.map(function(c) {
               return c.id;
             });
 
             // $FlowFixMe[missing-local-annot]
-            function recordModule(id: any, module) {
+            function recordModule(id: $FlowFixMe, 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.
@@ -235,7 +239,9 @@ export default class ReactFlightWebpackPlugin {
                 .getExportsInfo(module)
                 .getProvidedExports();
 
-              const moduleExports = {};
+              const moduleExports: {
+                [string]: {chunks: $FlowFixMe, id: $FlowFixMe, name: string},
+              } = {};
               ['', '*']
                 .concat(
                   Array.isArray(moduleProvidedExports)
@@ -352,7 +358,7 @@ export default class ReactFlightWebpackPlugin {
         result: $ReadOnlyArray<$ReadOnlyArray<ClientReferenceDependency>>,
       ): void => {
         if (err) return callback(err);
-        const flat = [];
+        const flat: Array<any> = [];
         for (let i = 0; i < result.length; i++) {
           // $FlowFixMe[method-unbinding]
           flat.push.apply(flat, result[i]);
diff --git a/packages/react-server/src/ReactFizzClassComponent.js b/packages/react-server/src/ReactFizzClassComponent.js
index f03117f65a440..599df4f6d1f97 100644
--- a/packages/react-server/src/ReactFizzClassComponent.js
+++ b/packages/react-server/src/ReactFizzClassComponent.js
@@ -20,8 +20,8 @@ import {REACT_CONTEXT_TYPE, REACT_PROVIDER_TYPE} from 'shared/ReactSymbols';
 import assign from 'shared/assign';
 import isArray from 'shared/isArray';
 
-const didWarnAboutNoopUpdateForComponent = {};
-const didWarnAboutDeprecatedWillMount = {};
+const didWarnAboutNoopUpdateForComponent: {[string]: boolean} = {};
+const didWarnAboutDeprecatedWillMount: {[string]: boolean} = {};
 
 let didWarnAboutUninitializedState;
 let didWarnAboutGetSnapshotBeforeUpdateWithoutDidUpdate;
diff --git a/packages/react-server/src/ReactFizzContext.js b/packages/react-server/src/ReactFizzContext.js
index 0a64c0997be24..48e2e20c32175 100644
--- a/packages/react-server/src/ReactFizzContext.js
+++ b/packages/react-server/src/ReactFizzContext.js
@@ -14,7 +14,7 @@ import checkPropTypes from 'shared/checkPropTypes';
 let warnedAboutMissingGetChildContext;
 
 if (__DEV__) {
-  warnedAboutMissingGetChildContext = {};
+  warnedAboutMissingGetChildContext = ({}: {[string]: boolean});
 }
 
 // $FlowFixMe[incompatible-exact]
@@ -32,7 +32,7 @@ export function getMaskedContext(type: any, unmaskedContext: Object): Object {
       return emptyContextObject;
     }
 
-    const context = {};
+    const context: {[string]: $FlowFixMe} = {};
     for (const key in contextTypes) {
       context[key] = unmaskedContext[key];
     }
diff --git a/packages/react-server/src/ReactFizzServer.js b/packages/react-server/src/ReactFizzServer.js
index bc7ece8424d12..d0ad4ec09c4bc 100644
--- a/packages/react-server/src/ReactFizzServer.js
+++ b/packages/react-server/src/ReactFizzServer.js
@@ -295,11 +295,11 @@ export function createRequest(
     completedRootSegment: null,
     abortableTasks: abortSet,
     pingedTasks: pingedTasks,
-    clientRenderedBoundaries: [],
-    completedBoundaries: [],
-    partialBoundaries: [],
-    preamble: [],
-    postamble: [],
+    clientRenderedBoundaries: ([]: Array<SuspenseBoundary>),
+    completedBoundaries: ([]: Array<SuspenseBoundary>),
+    partialBoundaries: ([]: Array<SuspenseBoundary>),
+    preamble: ([]: Array<Chunk | PrecomputedChunk>),
+    postamble: ([]: Array<Chunk | PrecomputedChunk>),
     onError: onError === undefined ? defaultErrorHandler : onError,
     onAllReady: onAllReady === undefined ? noop : onAllReady,
     onShellReady: onShellReady === undefined ? noop : onShellReady,
@@ -795,12 +795,12 @@ function renderClassComponent(
   popComponentStackInDEV(task);
 }
 
-const didWarnAboutBadClass = {};
-const didWarnAboutModulePatternComponent = {};
-const didWarnAboutContextTypeOnFunctionComponent = {};
-const didWarnAboutGetDerivedStateOnFunctionComponent = {};
+const didWarnAboutBadClass: {[string]: boolean} = {};
+const didWarnAboutModulePatternComponent: {[string]: boolean} = {};
+const didWarnAboutContextTypeOnFunctionComponent: {[string]: boolean} = {};
+const didWarnAboutGetDerivedStateOnFunctionComponent: {[string]: boolean} = {};
 let didWarnAboutReassigningProps = false;
-const didWarnAboutDefaultPropsOnFunctionComponent = {};
+const didWarnAboutDefaultPropsOnFunctionComponent: {[string]: boolean} = {};
 let didWarnAboutGenerators = false;
 let didWarnAboutMaps = false;
 let hasWarnedAboutUsingContextAsConsumer = false;
diff --git a/packages/react-server/src/ReactFlightServer.js b/packages/react-server/src/ReactFlightServer.js
index f8a320a09903d..acaece5541416 100644
--- a/packages/react-server/src/ReactFlightServer.js
+++ b/packages/react-server/src/ReactFlightServer.js
@@ -184,9 +184,9 @@ export function createRequest(
     pendingChunks: 0,
     abortableTasks: abortSet,
     pingedTasks: pingedTasks,
-    completedModuleChunks: [],
-    completedJSONChunks: [],
-    completedErrorChunks: [],
+    completedModuleChunks: ([]: Array<Chunk>),
+    completedJSONChunks: ([]: Array<Chunk>),
+    completedErrorChunks: ([]: Array<Chunk>),
     writtenSymbols: new Map(),
     writtenModules: new Map(),
     writtenProviders: new Map(),
diff --git a/packages/react/src/ReactChildren.js b/packages/react/src/ReactChildren.js
index 6208186fbf15d..6165ce2806dfb 100644
--- a/packages/react/src/ReactChildren.js
+++ b/packages/react/src/ReactChildren.js
@@ -249,7 +249,7 @@ function mapChildren(
   if (children == null) {
     return children;
   }
-  const result = [];
+  const result: Array<React$Node> = [];
   let count = 0;
   mapIntoArray(children, result, '', '', function(child) {
     return func.call(context, child, count++);
diff --git a/packages/react/src/ReactStartTransition.js b/packages/react/src/ReactStartTransition.js
index ef180ddaef423..490caf5d319e9 100644
--- a/packages/react/src/ReactStartTransition.js
+++ b/packages/react/src/ReactStartTransition.js
@@ -6,6 +6,7 @@
  *
  * @flow
  */
+import type {BatchConfigTransition} from 'react-reconciler/src/ReactFiberTracingMarkerComponent';
 import type {StartTransitionOptions} from 'shared/ReactTypes';
 
 import ReactCurrentBatchConfig from './ReactCurrentBatchConfig';
@@ -16,7 +17,7 @@ export function startTransition(
   options?: StartTransitionOptions,
 ) {
   const prevTransition = ReactCurrentBatchConfig.transition;
-  ReactCurrentBatchConfig.transition = {};
+  ReactCurrentBatchConfig.transition = ({}: BatchConfigTransition);
   const currentTransition = ReactCurrentBatchConfig.transition;
 
   if (__DEV__) {
@@ -40,6 +41,7 @@ export function startTransition(
     if (__DEV__) {
       if (prevTransition === null && currentTransition._updatedFibers) {
         const updatedFibersCount = currentTransition._updatedFibers.size;
+        currentTransition._updatedFibers.clear();
         if (updatedFibersCount > 10) {
           console.warn(
             'Detected a large number of updates inside startTransition. ' +
@@ -47,7 +49,6 @@ export function startTransition(
               'Otherwise concurrent mode guarantees are off the table.',
           );
         }
-        currentTransition._updatedFibers.clear();
       }
     }
   }
diff --git a/packages/scheduler/src/forks/SchedulerMock.js b/packages/scheduler/src/forks/SchedulerMock.js
index 6af3429b7fc25..8ce47fa4f97ad 100644
--- a/packages/scheduler/src/forks/SchedulerMock.js
+++ b/packages/scheduler/src/forks/SchedulerMock.js
@@ -66,7 +66,7 @@ var LOW_PRIORITY_TIMEOUT = 10000;
 var IDLE_PRIORITY_TIMEOUT = maxSigned31BitInt;
 
 // Tasks are stored on a min heap
-var taskQueue = [];
+var taskQueue: Array<Task> = [];
 var timerQueue: Array<Task> = [];
 
 // Incrementing id counter. Used to maintain insertion order.
diff --git a/packages/shared/checkPropTypes.js b/packages/shared/checkPropTypes.js
index 29d07f7afed18..aa2126f405286 100644
--- a/packages/shared/checkPropTypes.js
+++ b/packages/shared/checkPropTypes.js
@@ -7,7 +7,7 @@
  * @flow
  */
 
-const loggedTypeFailures = {};
+const loggedTypeFailures: {[string]: boolean} = {};
 
 import {describeUnknownElementTypeFrameInDEV} from 'shared/ReactComponentStackFrame';
 
diff --git a/scripts/error-codes/invertObject.js b/scripts/error-codes/invertObject.js
index 54a4b3743643c..f7a6428ef8d5d 100644
--- a/scripts/error-codes/invertObject.js
+++ b/scripts/error-codes/invertObject.js
@@ -16,8 +16,8 @@
  * into
  *   { 0: 'MUCH ERROR', 1: 'SUCH WRONG' }
  */
-function invertObject(targetObj /* : ErrorMap */) /* : ErrorMap */ {
-  const result = {};
+function invertObject(targetObj /*: ErrorMap */) /*: ErrorMap */ {
+  const result /*: {[string]: string} */ = {};
   const mapKeys = Object.keys(targetObj);
 
   // eslint-disable-next-line no-for-of-loops/no-for-of-loops
diff --git a/scripts/flow/config/flowconfig b/scripts/flow/config/flowconfig
index a4c50ad930eb6..ba0858e7c4163 100644
--- a/scripts/flow/config/flowconfig
+++ b/scripts/flow/config/flowconfig
@@ -44,6 +44,7 @@ untyped-type-import=error
 exact_by_default=true
 munge_underscores=false
 enforce_local_inference_annotations=true
+exact_empty_objects=true
 
 # Substituted by createFlowConfig.js:
 %REACT_RENDERER_FLOW_OPTIONS%