transform-async-generator-functions not transform for-await in async functions

[hax]

Input Code

async function foo() {
	for await (const x of y) {}
}
async function* bar() {
	for await (const x of y) {}
}

Babel Configuration (.babelrc, package.json, cli command)

{
	"plugins": [
		"transform-async-generator-functions"
	],
}

Expected Behavior

Both bar() and for-await-of in foo() should be transformed.

Current Behavior

for-await-of in foo() is not transformed.

Possible Solution

As I understand, for await (const x of y) { x } is like a sugar of: for (const x of y) { await x } (but call y[Symbol.asyncIterator]() instead of y[Symbol.iterator]())
Maybe we can introduce a separate plugin transform-for-await-of.

[Jamesernator]

It looks like the logic for converting for-await loops is actually in remap-async-to-generator for some reason, which is a pain because it means that in order to use for-await in an async function you have to also convert them into generators.

I might have a look this week into moving it into its own helper and make async-generator-functions use it.

[Jessidhia]

What's actually a Promise, in async iterators, is the the iterator result object itself, which is why new syntax, symbols, etc were required.

for await is actually closer to a sugar of:

// _very_ incomplete sugar
for (
  let iter = y[Symbol.asyncIterator](), step = await iter.next();
  !step.done;
  step = await iter.next()
) {
  const x = step.value // note, no await here!
}

[iamchenxin]

@Jamesernator Does "transform-async-generator-functions" work well now? I test with '6.24.1', still do not work. And it seems it will transform 'for await' to 'generators' with an additional plugin 'transform-async-to-generator'.

[alexeyraspopov]

#7446 is merged. Should this issue be closed?

[existentialism]

@alexeyraspopov thanks!

[alexeyraspopov]

@existentialism, not sure about proper place to ask, but do you know what else should happen so the babel-preset-env will start supporting async generators (and object rest/spread I guess)?

[mnpenner]

How do I 'get' this fix?

My .babelrc:

{
    "plugins": [
        "transform-object-rest-spread",
        "transform-class-properties",
        "transform-function-bind",
        "syntax-async-generators",
        "transform-async-generator-functions"
    ],
    "presets": [
        [
            "env",
            {
                "targets": {
                    "node": "current"
                }
            }
        ]
    ],
    "compact": false
}

And package.json:

  "devDependencies": {
    "babel-cli": "^6.26.0",
    "babel-plugin-syntax-async-generators": "^6.13.0",
    "babel-plugin-transform-async-generator-functions": "^6.24.1",
    "babel-plugin-transform-class-properties": "^6.24.1",
    "babel-plugin-transform-function-bind": "^6.22.0",
    "babel-plugin-transform-object-rest-spread": "^6.26.0",
    "babel-preset-env": "^1.6.1"
  }

What versions do I have to bump? The fix is on npm, right?

[nicolo-ribaudo]

It's in Babel 7 beta.