[Compiler]: How to mark valtio's `useProxy`

[nmay231]

What kind of issue is this?

• React Compiler core (the JS output is incorrect, or your app works incorrectly after optimization)
• babel-plugin-react-compiler (build issue installing or using the Babel plugin)
• eslint-plugin-react-compiler (build issue installing or using the eslint plugin)
• react-compiler-healthcheck (build issue installing or using the healthcheck script)

Link to repro

https://github.com/nmay231/eslint-plugin-react-compiler-bug

Repro steps

How to see issue with given repo:

1. Clone repo above.
2. Click the optimized button 5+ times. The child component stops rerendering since the prop no longer changes
3. Click the unoptimized button 5+ times. The child component keeps rerendering even though the function is safe to memoize.

When you pass a valtio proxy object to useProxy, it is safe to modify the resulting proxy object directly as a way to setState. Peeking at Unoptimized.tsx, in particular the button onClick handler:

export function Unoptimized() {
    const state = useProxy(stateProxy)
    return (
        <>
            <p>Unoptimized</p>

            {/* Child continues to rerender even though props are the same */}
            <ChildComponent clicks={Math.min(state.clicks, 5)} />

            <button
                onClick={() => {
                    // Lint error: Mutating a value returned from a function whose return value should not be mutated
                    state.attr = !state.attr
                    stateProxy.clicks += 1
                    console.log(stateProxy.clicks)
                }}>
                Clicked {state.clicks}
            </button>
        </>
    )
}

The problem

The compiler (and eslint plugin) fail to realize this is safe to optimize, aka this is a false negative. One workaround is to use const state = useSnapshot(myProxy) for reading from the proxy and to modify the proxy object directly (myProxy.attr = ...). Another is to manually optimize with React.memo where proxy objects are used, but that feels counter-productive.

The real question is, Is there a way for valtio developers to mark useProxy's return value as safe to mutate?

Thanks.

Steps to recreate repo from scratch

1. pnpm create vite
2. pnpm add -D eslint-plugin-react-compiler@beta
3. pnpm add react-compiler-runtime@beta babel-plugin-react-compiler@beta valtio
4. Update eslint config to use plugin, and vite.config.ts to use babel plugin (skimming though this guide helped me).
5. Create a proxy object, and use the proxy object using valtio's useProxy hook.
6. Modifying the proxy object should be safe to do, but the compiler rules are too simple to realize that it is safe.

How often does this bug happen?

Every time

What version of React are you using?

18.3.1

What version of React Compiler are you using?

19.0.0-beta-8a03594-20241020

[josephsavona]

Thanks for posting. It looks like the useProxy() API specifically is breaking React’s rules. Normal Valtio usage per the website should be fine - w proxy() calls outside components and then useSnapshot() to get an immutable snapshot for rendering.

The problem is specifically with useProxy(), which is a mutable value. This is probably worth posting about as a Valtio issue.

[nmay231]

Fair enough. And as mentioned, I can just use useSnapshot() to follow the rules or manually memoize.

I do have a new question. I can open a new issue for it if it turns out to not be simple. Can you see which components are not optimized and why? I am using the eslint plugin and see 5 components with errors (they all used useProxy), but running npx react-compiler-healthcheck@beta says 35/44 components optimized. Perhaps a --verbose flag would be helpful.

Let me know if I should open an issue for that question. I'll close this for now.

EDIT: I see #29080 now, haha