AsyncGenerator: does not implement [Symbol.asyncIterator]

[mfulton26]

Bug Description

AsyncGenerator instances in Hermes do not conform to the async iterable protocol as they do not implement [Symbol.asyncIterator]

• I have run gradle clean and confirmed this bug does not occur with JSC

Hermes version: (How do I look this up? I'm using whatever version I would have after follow these instructions. 0.70.1?)
React Native version (if any): 0.70.1
OS version (if any):
Platform (most likely one of arm64-v8a, armeabi-v7a, x86, x86_64):

Steps To Reproduce

console.log(Symbol.asyncIterator in (async function* () {})());
console.log(Symbol.iterator in (function* () {})());

The Expected Behavior

 LOG  true
 LOG  true

The Actual Behavior

 LOG  false
 LOG  true

[tmikov]

Hi,
Thank you for reporting this. Hermes doesn't support async generators, which were introduced in ES2018. If I try to compile your example, I get:

bug.js:1:38: error: async generators are unsupported
console.log(Symbol.asyncIterator in (async function* () {})());
                                     ^~~~~~~~~~~~~~~~~~~~~

We will add them to Hermes, but at this time we don't have a concrete timeline.

[mfulton26]

I did not realize that async generators were introduced after generators; thank you for providing that clarity

I didn't see any compilation errors in my React Native project (at build time nor at run time); that might be a separate issue in and of itself

thank you

[tmikov]

There is probably a Babel pass for async generators that runs first, so Hermes doesn't see them at all. It is possible that the Babel transform is not fully spec compliant, but still usable?

[tmikov]

This will be implemented when we transition to support the corresponding version of EcmaScript.