React Compiler compatibility module for codebases unable to use React 19

babel.config.js

module.exports = function(api) {
  return {
    plugins: [
      [
        'babel-plugin-react-compiler', {
          runtimeModule: 'react-compiler-runtime'
        }
      ]
    ]
  }
}

package.json

{
  "dependencies": {
    "react-compiler-runtime": "file:./lib/react-compiler-runtime"
  },
  "devDependencies": {
    "babel-plugin-react-compiler": "latest"
  }
}

react-compiler-runtime-index.js

// lib/react-compiler-runtime/index.js

const React = require('react');
const $empty = Symbol.for("react.memo_cache_sentinel");
/**
 * DANGER: this hook is NEVER meant to be called directly!
 *
 * Note that this is a temporary userspace implementation of this function
 * from React 19. It is not as efficient and may invalidate more frequently
 * than the official API. Please upgrade to React 19 as soon as you can.
 **/
export function c(size: number) {
  return React.useState(() => {
    const $ = new Array(size);
    for (let ii = 0; ii < size; ii++) {
      $[ii] = $empty;
    }
    // @ts-ignore
    $[$empty] = true;
    return $;
  })[0];
}

react-compiler-runtime-package.json

// lib/react-compiler-runtime/package.json
{
  "name": "react-compiler-runtime",
  "version": "0.0.1",
  "license": "MIT",
  "main": "index.js",
  "dependencies": {
    "react": "^18.2.0"
  }
}

The polyfill doesn't seem to work on React >18. We get

Error: Missing "./compiler-runtime" specifier in "react" package
    at e 

Works perfect in React 17.0.2 though

make sure you are not using the "4.3.0" version @vitejs/plugin-react, try to use "4.2.1"

@iahu
downgrading vite plugin worked! cheers

#file-react-compiler-runtime-index-js-L13
What's the difference between using React.useState and returning an normal Array ?
I tried Array to work as well too

+1ing @jsonz1993

Middle ground may even be using useRef to keep it safely contained and consistent.

// lib/react-compiler-runtime/index.js

const { useRef } = require('react');
const $empty = Symbol.for("react.memo_cache_sentinel");
/**
 * DANGER: this hook is NEVER meant to be called directly!
 *
 * Note that this is a temporary userspace implementation of this function
 * from React 19. It is not as efficient and may invalidate more frequently
 * than the official API. Please upgrade to React 19 as soon as you can.
 **/
export function c(size: number): any[] {
  const memoArrayRef = useRef<any[] | undefined>(undefined)

  if (!memoArrayRef.value) {
    memoArrayRef.value = []
    for (let i = 0; i < size; i += 1) {
      memoArrayRef.value.push($empty)
    }
    // @ts-ignore
    memoArrayRef.value[$empty] = true;
  }

  return memoArrayRef.value
}

@poteto I'm curious, why does the polyfill appear to return the first element of the created array instead of the array itself? surely that is a mistake?

I'm curious, why does the polyfill appear to return the first element of the created array instead of the array itself? surely that is a mistake?

@jaeschliman With the react function useState, it returns a tuple containing 2 values. The first one is the state, the second one is a setter for the state. We are only interested in the state from the useState and discard the setter. Note that it is rare to not use the setter, most of the time when you use useState, you actually want to be able to change the state, not just here

More reading of the useState hook: https://react.dev/reference/react/useState

@ferrybig ah yes, of course — I am so used to seeing it written with the destructuring syntax that I was blind to that — thanks