"for await" inside async functions

[LeviticusMB]

With PR #11326 now merged, isn't this supposed to work?

TypeScript Version:
Version 2.3.0-dev.20170217

Node version
v6.9.1

Command line

tsc --target es6 --lib esnext --outFile for-await.js for-await.ts ; node for-await.js

Code

function delay(ms) {
    return new Promise((resolve, reject) => {
        setTimeout(resolve, ms);
    });
}

async function* numbers() {
    yield 1;
    await delay(500);
    yield 2;
    await delay(1000);
    yield 3;
    await delay(2000);
    yield 4;
}

(async () => {
    let n = numbers();
    console.log(await n.next());
    console.log(await n.next());
    console.log(await n.next());
    console.log(await n.next());
    console.log(await n.next());

    for await (let next of numbers()) {
        console.log(next);
    }
})();

Expected behavior:

{ value: 1, done: false }
{ value: 2, done: false }
{ value: 3, done: false }
{ value: 4, done: false }
{ value: undefined, done: true }
1
2
3
4

Actual behavior:

{ value: 1, done: false }
{ value: 2, done: false }
{ value: 3, done: false }
{ value: 4, done: false }
{ value: undefined, done: true }
undefined
undefined
undefined
undefined
undefined
undefined
undefined
undefined
undefined
...

[rbuckton]

There are two underlying issues here:

1. The for-await-of transformation isn't emitting the correct await downleveling for each step.
2. The second is that you do not have Symbol.asyncIterator defined, and we are not reporting an error at runtime for this.

I'm working on a fix for (1) and part of (2) now. Afterwards you will need to ensure you have a shim for Symbol.asyncIterator, for example:

(<any>Symbol).asyncIterator = Symbol.asyncIterator || Symbol.for("Symbol.asyncIterator");

TypeScript does not currently plan to emit this shim itself, as it could collide with existing shim libraries.

[LeviticusMB]

Awesome, thanks! 👍