Partial async generator support present in Hermes isn't spec compliant

leotm commented 3 weeks ago

Bug Description

[ ] I have run gradle clean and confirmed this bug does not occur with JSC
[x] The issue is reproducible with the latest version of React Native.

Hermes git revision (if applicable): hermes-2023-03-07-RNv0.71.4-ee25883ea34374f687883a641c8101ac0d292fc6 React Native version: 71.17 OS: Android (running on macOS) Platform (most likely one of arm64-v8a, armeabi-v7a, x86, x86_64): arm64-v8a

Steps To Reproduce

Attempt to access async iterator proto

code example:

async function* AsyncGeneratorFunctionInstance() {}
const AsyncGeneratorFunction = Object.getPrototypeOf(
  AsyncGeneratorFunctionInstance,
).constructor
const AsyncGenerator = AsyncGeneratorFunction.prototype;

const asyncIterable = {
  async *[Symbol.asyncIterator]() {},
};
var results = [
  typeof asyncIterable[Symbol.asyncIterator].prototype.__proto__ === 'object', // it's an object even in Hermes, but looks a different shape
  typeof AsyncGenerator.prototype, // undefined on hermes
  asyncIterable[Symbol.asyncIterator].prototype.__proto__ === AsyncGenerator.prototype, // true in other engines
  asyncIterable[Symbol.asyncIterator].prototype.__proto__.__proto__ // null in Hermes, object with an iterator fn in other engines
];

// Node 18.18.2
// console.log(Object.getOwnPropertyDescriptors(asyncIterable[Symbol.asyncIterator].prototype.__proto__.__proto__))
// {
//   [Symbol(Symbol.asyncIterator)]: {
//     value: [Function: [Symbol.asyncIterator]],
//     writable: true,
//     enumerable: false,
//     configurable: true
//   }
// }

throw JSON.stringify(results);

The Expected Behavior

Other engines: [true,"object",true,{}] Hermes: [true,"undefined",false,null]

tmikov commented 2 weeks ago

Sorry, Hermes has no async iterator support at the moment (though we will add it to Static Hermes). I think you are essentially testing a Babel transformation.

leotm commented 2 weeks ago

thanks for clarifying ^ and great news it will be in Static Hermes 🎉

my mistaken assumption was node_modules/react-native/Libraries/Core/InitializeCore.js is running Babel-free

for anyone else to run in the React Native environment w/o Babel you could

new Function('{var}', String.raw`
  // my code (should be same result as Hermes CLI/REPL)
`)({var: 1});

or the usual way

// babel.config.js
module.exports = {
  ignore: [/myFile\.cjs/],
  presets: ['module:metro-react-native-babel-preset'],
};

or outside the React Native environment, directly via the Hermes CLI or building the static_h branch for latest

otherwise yes in this example we can see inspecting the bundle Babel transforms with few helpers

@babel/runtime/helpers/AsyncGenerator
@babel/runtime/helpers/OverloadYield
@babel/runtime/helpers/wrapAsyncGenerator

coming from various transform plugins in the preset

Babel REPL example with preset: here

facebook / hermes