[React 19]: `react-compiler-runtime` always polyfills, even in React 19

[SimenB]

Summary

https://github.com/SimenB/react-19-compiler-runtime

Using react-compiler-runtime always loads the polyfill. Probably because react does not have the __COMPILER_RUNTIME export it's looking for.

[poteto]

Could you clarify the usecase? I ran your script and got this, which looks correct to me:

polyfill:
 function c(size) {
        return React__namespace.useMemo(() => {
            const $ = new Array(size);
            for (let ii = 0; ii < size; ii++) {
                $[ii] = $empty;
            }
            $[$empty] = true;
            return $;
        }, []);
    }



real:
 function (size) {
      var dispatcher = ReactSharedInternals.H;
      null === dispatcher &&
        console.error(
          "Invalid hook call. Hooks can only be called inside of the body of a function component. This could happen for one of the following reasons:\n1. You might have mismatching versions of React and the renderer (such as React DOM)\n2. You might be breaking the Rules of Hooks\n3. You might have more than one copy of React in the same app\nSee https://react.dev/link/invalid-hook-call for tips about how to debug and fix this problem."
        );
      return dispatcher.useMemoCache(size);
    }

[SimenB]

I might have misunderstood, but I thought the react-compiler-runtime package should essentially just pass through to react (react/compiler-runtime, which is the import without the target field in the babel plugin) in versions 19 and above and not have a runtime effect?

From the docs: https://react.dev/learn/react-compiler#using-react-compiler-with-react-17-or-18

The runtime package will use the correct implementation of APIs depending on the application’s version, and polyfill the missing APIs if necessary.

The package currently always polyfills, not just "if necessary". The code comment also indicates that it's supposed to use a built in version for React 19, and only polyfill older versions:

react/compiler/packages/react-compiler-runtime/src/index.ts

Lines 23 to 24 in 380f5d6

	// Re-export React.c if present, otherwise fallback to the userspace polyfill for versions of React
	// < 19.

[poteto]

Oh right sorry, I misunderstood! Let me take a look and see what's going on

[poteto]

I see what's going on. Our React 19 RC/Beta releases currently only build in the stable channel, and I only exported the __COMPILER_RUNTIME in non-stable. I have a PR to promote: #31540

In the meantime you can use one of the experimental builds which should have it exported. Could you give that a try and let me know if you still see the issue?

[SimenB]

Workaround I'm using is to map react-compiler-runtime to react/compiler-runtime in the bundler (for both webpack and vite in different projects).

But yeah, I can see React.__COMPILER_RUNTIME in the experimental version 👍

[SimenB]

Fixed by linked PR 🎉 Thanks @poteto!