From bfd8da807c75a2d123627415f9eaf2d36ac3ed6a Mon Sep 17 00:00:00 2001
From: Andrew Clark <git@andrewclark.io>
Date: Fri, 5 Apr 2024 13:06:39 -0400
Subject: [PATCH] Make class prop resolution faster (#28766)

`delete` causes an object (in V8, and maybe other engines) to deopt to a
dictionary instead of a class. Instead of `assign` + `delete`, manually
iterate over all the properties, like the JSX runtime does.

To avoid copying the object twice I moved the `ref` prop removal to come
before handling default props. If we already cloned the props to remove
`ref`, then we can skip cloning again to handle default props.
---
 .../src/ReactFiberClassComponent.js           | 31 +++++++++------
 packages/react-server/src/ReactFizzServer.js  | 38 ++++++++++++-------
 2 files changed, 44 insertions(+), 25 deletions(-)

diff --git a/packages/react-reconciler/src/ReactFiberClassComponent.js b/packages/react-reconciler/src/ReactFiberClassComponent.js
index e464670bfedb4..f802c261ba418 100644
--- a/packages/react-reconciler/src/ReactFiberClassComponent.js
+++ b/packages/react-reconciler/src/ReactFiberClassComponent.js
@@ -1251,7 +1251,19 @@ export function resolveClassComponentProps(
 ): Object {
   let newProps = baseProps;
 
-  // Resolve default props. Taken from old JSX runtime, where this used to live.
+  if (enableRefAsProp) {
+    // Remove ref from the props object, if it exists.
+    if ('ref' in baseProps) {
+      newProps = ({}: any);
+      for (const propName in baseProps) {
+        if (propName !== 'ref') {
+          newProps[propName] = baseProps[propName];
+        }
+      }
+    }
+  }
+
+  // Resolve default props.
   const defaultProps = Component.defaultProps;
   if (
     defaultProps &&
@@ -1259,7 +1271,12 @@ export function resolveClassComponentProps(
     // default props here in the reconciler, rather than in the JSX runtime.
     (disableDefaultPropsExceptForClasses || !alreadyResolvedDefaultProps)
   ) {
-    newProps = assign({}, newProps, baseProps);
+    // We may have already copied the props object above to remove ref. If so,
+    // we can modify that. Otherwise, copy the props object with Object.assign.
+    if (newProps === baseProps) {
+      newProps = assign({}, newProps, baseProps);
+    }
+    // Taken from old JSX runtime, where this used to live.
     for (const propName in defaultProps) {
       if (newProps[propName] === undefined) {
         newProps[propName] = defaultProps[propName];
@@ -1267,16 +1284,6 @@ export function resolveClassComponentProps(
     }
   }
 
-  if (enableRefAsProp) {
-    // Remove ref from the props object, if it exists.
-    if ('ref' in newProps) {
-      if (newProps === baseProps) {
-        newProps = assign({}, newProps);
-      }
-      delete newProps.ref;
-    }
-  }
-
   return newProps;
 }
 
diff --git a/packages/react-server/src/ReactFizzServer.js b/packages/react-server/src/ReactFizzServer.js
index 327311d74fa31..a322d57df695f 100644
--- a/packages/react-server/src/ReactFizzServer.js
+++ b/packages/react-server/src/ReactFizzServer.js
@@ -1397,24 +1397,36 @@ export function resolveClassComponentProps(
 ): Object {
   let newProps = baseProps;
 
-  // Resolve default props. Taken from old JSX runtime, where this used to live.
-  const defaultProps = Component.defaultProps;
-  if (defaultProps && disableDefaultPropsExceptForClasses) {
-    newProps = assign({}, newProps, baseProps);
-    for (const propName in defaultProps) {
-      if (newProps[propName] === undefined) {
-        newProps[propName] = defaultProps[propName];
+  if (enableRefAsProp) {
+    // Remove ref from the props object, if it exists.
+    if ('ref' in baseProps) {
+      newProps = ({}: any);
+      for (const propName in baseProps) {
+        if (propName !== 'ref') {
+          newProps[propName] = baseProps[propName];
+        }
       }
     }
   }
 
-  if (enableRefAsProp) {
-    // Remove ref from the props object, if it exists.
-    if ('ref' in newProps) {
-      if (newProps === baseProps) {
-        newProps = assign({}, newProps);
+  // Resolve default props.
+  const defaultProps = Component.defaultProps;
+  if (
+    defaultProps &&
+    // If disableDefaultPropsExceptForClasses is true, we always resolve
+    // default props here, rather than in the JSX runtime.
+    disableDefaultPropsExceptForClasses
+  ) {
+    // We may have already copied the props object above to remove ref. If so,
+    // we can modify that. Otherwise, copy the props object with Object.assign.
+    if (newProps === baseProps) {
+      newProps = assign({}, newProps, baseProps);
+    }
+    // Taken from old JSX runtime, where this used to live.
+    for (const propName in defaultProps) {
+      if (newProps[propName] === undefined) {
+        newProps[propName] = defaultProps[propName];
       }
-      delete newProps.ref;
     }
   }