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Performance: remove memory allocations in useReducer #33165

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Performance: remove memory allocations in useReducer #33165

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20 changes: 12 additions & 8 deletions packages/react-server/src/ReactFizzHooks.js
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Original file line number Diff line number Diff line change
Expand Up @@ -379,7 +379,7 @@ export function useReducer<S, I, A>(
// $FlowFixMe[incompatible-use] found when upgrading Flow
renderPhaseUpdates.delete(queue);
// $FlowFixMe[incompatible-use] found when upgrading Flow
let newState = workInProgressHook.memoizedState;
let newState = workInProgressHook.memoizedState[0];
let update: Update<any> = firstRenderPhaseUpdate;
do {
// Process this render phase update. We don't have to check the
Expand All @@ -398,13 +398,14 @@ export function useReducer<S, I, A>(
} while (update !== null);

// $FlowFixMe[incompatible-use] found when upgrading Flow
workInProgressHook.memoizedState = newState;

return [newState, dispatch];
workInProgressHook.memoizedState = [newState, dispatch];
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@romgrk romgrk May 9, 2025 •
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I thought about .memoizedState[0] = newState, but that would make the return value of useState stable across renders, even when there are changes, and that would be conceptually wrong for this API.

Tbh my true aim was to have a state hook with a stable value (and absolutely no memory allocations beyond initialization), but it would have an API like this:

const enabled = useStableState(false);
// => { get: () => boolean, set: (v: boolean) => void }

return (
  <button onClick=(() => enabled.set(!enabled.get())>
    {enabled.get() ? 'enabled' : 'disabled'}
  </button>
)

However, I fear it won't be possible without adding it directly in React as I need access to the internals to implement it properly.

if (__DEV__) {
Object.freeze(workInProgressHook.memoizedState);
}
}
}
// $FlowFixMe[incompatible-use] found when upgrading Flow
return [workInProgressHook.memoizedState, dispatch];
return workInProgressHook.memoizedState;
} else {
if (__DEV__) {
isInHookUserCodeInDev = true;
Expand All @@ -424,8 +425,6 @@ export function useReducer<S, I, A>(
isInHookUserCodeInDev = false;
}
// $FlowFixMe[incompatible-use] found when upgrading Flow
workInProgressHook.memoizedState = initialState;
// $FlowFixMe[incompatible-use] found when upgrading Flow
const queue: UpdateQueue<A> = (workInProgressHook.queue = {
last: null,
dispatch: null,
Expand All @@ -436,7 +435,12 @@ export function useReducer<S, I, A>(
queue,
): any));
// $FlowFixMe[incompatible-use] found when upgrading Flow
return [workInProgressHook.memoizedState, dispatch];
workInProgressHook.memoizedState = [initialState, dispatch];
if (__DEV__) {
Object.freeze(workInProgressHook.memoizedState);
}
// $FlowFixMe[incompatible-use] found when upgrading Flow
return workInProgressHook.memoizedState;
}
}

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