Close sync iterables when they yield rejected promises

[js-choi]

Previously, for await did not close sync iterables when they yield rejected promises (tc39/ecma262#1849):

Code example

function * createIter() {
  try {
    yield Promise.resolve(console.log("a"));
    yield Promise.reject("x");
  } finally {
    console.log("finalized");
  }
}

// Prints "a" and then prints "finalized".
// There is an uncaught "x" rejection.
for (const x of createIter()) {
  console.log(await x);
}

// Prints "a" and then prints "finalized".
// There is an uncaught "x" rejection.
Array.from(createIter());

A month ago, the editors merged tc39/ecma262#2600 into the language. Now, for await closes sync iterables when they yield rejected promises. Array.fromAsync needs to change to do the same. See #39.

• Update spec
• Update explainer section
• Update test262
• Open an issue on MDN

[bakkot]

It actually doesn't need any changes to the spec: Array.fromAsync uses CreateAsyncFromSyncIterator, and that's where the change happened.

Specifically, the Let nextResult be ? Call(iteratorRecord.[[NextMethod]], iteratorRecord.[[Iterator]]). step will call %AsyncFromSyncIteratorPrototype%.next, which calls AsyncFromSyncIteratorContinuation passing true for closeOnRejection, which causes it to close the underlying sync iterable when a rejected promises is yielded (as part of the rejection handler specified in step 13.a).

However, the explainer does indeed need to be updated. Also tests should be updated (or written).

[js-choi]

A test262 pull request is pending (tc39/test262#4450).

@bakkot: I see a “Note about changed behavior of Array.fromAsync after landing #2600” item in the 2025-04 plenary agenda. Is there anything I need to do on my end regarding this agenda item?

[bakkot]

I had genuinely forgotten I'd added that! But no, nothing from you; that item is just to inform engines that this change to fromAsync had happened as a result of the 262 editors merging the old normative PR to 262.

[js-choi]

Issue to change MDN article on Array.fromAsync and for await (… of …) tracked in mdn/content#39114 mdn/content#37038.