Fast JSX: Don't clone props object (#28768)

(Unless "key" is spread onto the element.)

Historically, the JSX runtime clones the props object that is passed in.
We've done this for two reasons.

One reason is that there are certain prop names that are reserved by
React, like `key` and (before React 19) `ref`. These are not actual
props and are not observable by the target component; React uses them
internally but removes them from the props object before passing them to
userspace.

The second reason is that the classic JSX runtime, `createElement`, is
both a compiler target _and_ a public API that can be called manually.
Therefore, we can't assume that the props object that is passed into
`createElement` won't be mutated by userspace code after it is passed
in.

However, the new JSX runtime, `jsx`, is not a public API — it's solely a
compiler target, and the compiler _will_ always pass a fresh, inline
object. So the only reason to clone the props is if a reserved prop name
is used.

In React 19, `ref` is no longer a reserved prop name, and `key` will
only appear in the props object if it is spread onto the element.
(Because if `key` is statically defined, the compiler will pass it as a
separate argument to the `jsx` function.) So the only remaining reason
to clone the props object is if `key` is spread onto the element, which
is a rare case, and also triggers a warning in development.

In a future release, we will not remove a spread key from the props
object. (But we'll still warn.) We'll always pass the object straight
through.

The expected impact is much faster JSX element creation, which in many
apps is a significant slice of the overall runtime cost of rendering.

packages/react/src/__tests__/ReactJSXRuntime-test.js

@@ -374,4 +374,37 @@ describe('ReactJSXRuntime', () => {
     }
     expect(didCall).toBe(false);
   });
+
+  // @gate enableRefAsProp
+  // @gate disableStringRefs
+  it('does not clone props object if key is not spread', async () => {
+    const config = {
+      foo: 'foo',
+      bar: 'bar',
+    };
+
+    const element = __DEV__
+      ? JSXDEVRuntime.jsxDEV('div', config)
+      : JSXRuntime.jsx('div', config);
+    expect(element.props).toBe(config);
+
+    const configWithKey = {
+      foo: 'foo',
+      bar: 'bar',
+      // This only happens when the key is spread onto the element. A statically
+      // defined key is passed as a separate argument to the jsx() runtime.
+      key: 'key',
+    };
+
+    let elementWithSpreadKey;
+    expect(() => {
+      elementWithSpreadKey = __DEV__
+        ? JSXDEVRuntime.jsxDEV('div', configWithKey)
+        : JSXRuntime.jsx('div', configWithKey);
+    }).toErrorDev(
+      'A props object containing a "key" prop is being spread into JSX',
+      {withoutStack: true},
+    );
+    expect(elementWithSpreadKey.props).not.toBe(configWithKey);
+  });
 });

packages/react/src/jsx/ReactJSXElement.js

@@ -314,11 +314,6 @@ function ReactElement(type, key, _ref, self, source, owner, props) {
  * @param {string} key
  */
 export function jsxProd(type, config, maybeKey) {
-  let propName;
-
-  // Reserved names are extracted
-  const props = {};
-
   let key = null;
   let ref = null;
 
@@ -351,22 +346,39 @@ export function jsxProd(type, config, maybeKey) {
     }
   }
 
-  // Remaining properties are added to a new props object
-  for (propName in config) {
-    if (
-      hasOwnProperty.call(config, propName) &&
-      // Skip over reserved prop names
-      propName !== 'key' &&
-      (enableRefAsProp || propName !== 'ref')
-    ) {
-      if (enableRefAsProp && !disableStringRefs && propName === 'ref') {
-        props.ref = coerceStringRef(
-          config[propName],
-          ReactCurrentOwner.current,
-          type,
-        );
-      } else {
-        props[propName] = config[propName];
+  let props;
+  if (enableRefAsProp && disableStringRefs && !('key' in config)) {
+    // If key was not spread in, we can reuse the original props object. This
+    // only works for `jsx`, not `createElement`, because `jsx` is a compiler
+    // target and the compiler always passes a new object. For `createElement`,
+    // we can't assume a new object is passed every time because it can be
+    // called manually.
+    //
+    // Spreading key is a warning in dev. In a future release, we will not
+    // remove a spread key from the props object. (But we'll still warn.) We'll
+    // always pass the object straight through.
+    props = config;
+  } else {
+    // We need to remove reserved props (key, prop, ref). Create a fresh props
+    // object and copy over all the non-reserved props. We don't use `delete`
+    // because in V8 it will deopt the object to dictionary mode.
+    props = {};
+    for (const propName in config) {
+      if (
+        hasOwnProperty.call(config, propName) &&
+        // Skip over reserved prop names
+        propName !== 'key' &&
+        (enableRefAsProp || propName !== 'ref')
+      ) {
+        if (enableRefAsProp && !disableStringRefs && propName === 'ref') {
+          props.ref = coerceStringRef(
+            config[propName],
+            ReactCurrentOwner.current,
+            type,
+          );
+        } else {
+          props[propName] = config[propName];
+        }
       }
     }
   }
@@ -375,7 +387,7 @@ export function jsxProd(type, config, maybeKey) {
     // Resolve default props
     if (type && type.defaultProps) {
       const defaultProps = type.defaultProps;
-      for (propName in defaultProps) {
+      for (const propName in defaultProps) {
         if (props[propName] === undefined) {
           props[propName] = defaultProps[propName];
         }
@@ -538,11 +550,6 @@ export function jsxDEV(type, config, maybeKey, isStaticChildren, source, self) {
       }
     }
 
-    let propName;
-
-    // Reserved names are extracted
-    const props = {};
-
     let key = null;
     let ref = null;
 
@@ -578,22 +585,39 @@ export function jsxDEV(type, config, maybeKey, isStaticChildren, source, self) {
       }
     }
 
-    // Remaining properties are added to a new props object
-    for (propName in config) {
-      if (
-        hasOwnProperty.call(config, propName) &&
-        // Skip over reserved prop names
-        propName !== 'key' &&
-        (enableRefAsProp || propName !== 'ref')
-      ) {
-        if (enableRefAsProp && !disableStringRefs && propName === 'ref') {
-          props.ref = coerceStringRef(
-            config[propName],
-            ReactCurrentOwner.current,
-            type,
-          );
-        } else {
-          props[propName] = config[propName];
+    let props;
+    if (enableRefAsProp && disableStringRefs && !('key' in config)) {
+      // If key was not spread in, we can reuse the original props object. This
+      // only works for `jsx`, not `createElement`, because `jsx` is a compiler
+      // target and the compiler always passes a new object. For `createElement`,
+      // we can't assume a new object is passed every time because it can be
+      // called manually.
+      //
+      // Spreading key is a warning in dev. In a future release, we will not
+      // remove a spread key from the props object. (But we'll still warn.) We'll
+      // always pass the object straight through.
+      props = config;
+    } else {
+      // We need to remove reserved props (key, prop, ref). Create a fresh props
+      // object and copy over all the non-reserved props. We don't use `delete`
+      // because in V8 it will deopt the object to dictionary mode.
+      props = {};
+      for (const propName in config) {
+        if (
+          hasOwnProperty.call(config, propName) &&
+          // Skip over reserved prop names
+          propName !== 'key' &&
+          (enableRefAsProp || propName !== 'ref')
+        ) {
+          if (enableRefAsProp && !disableStringRefs && propName === 'ref') {
+            props.ref = coerceStringRef(
+              config[propName],
+              ReactCurrentOwner.current,
+              type,
+            );
+          } else {
+            props[propName] = config[propName];
+          }
         }
       }
     }
@@ -602,7 +626,7 @@ export function jsxDEV(type, config, maybeKey, isStaticChildren, source, self) {
       // Resolve default props
       if (type && type.defaultProps) {
         const defaultProps = type.defaultProps;
-        for (propName in defaultProps) {
+        for (const propName in defaultProps) {
           if (props[propName] === undefined) {
             props[propName] = defaultProps[propName];
           }