ReadableStream async iterator polyfill

readable-stream-async-iterator-polyfill.js

/**
 * A polyfill for `ReadableStream.protototype[Symbol.asyncIterator]`,
 * aligning as closely as possible to the specification.
 *
 * @see https://streams.spec.whatwg.org/#rs-asynciterator
 * @see https://developer.mozilla.org/en-US/docs/Web/API/ReadableStream#async_iteration
 */
ReadableStream.prototype.values ??= function({ preventCancel = false } = {}) {
  const reader = this.getReader();
  return {
    async next() {
      try {
        const result = await reader.read();
        if (result.done) {
          reader.releaseLock();
        }
        return result;
      } catch (e) {
        reader.releaseLock();
        throw e;
      }
    },
    async return(value) {
      if (!preventCancel) {
        const cancelPromise = reader.cancel(value);
        reader.releaseLock();
        await cancelPromise;
      } else {
        reader.releaseLock();
      }
      return { done: true, value };
    },
    [Symbol.asyncIterator]() {
      return this;
    }
  };
};

ReadableStream.prototype[Symbol.asyncIterator] ??= ReadableStream.prototype.values;

@MattiasBuelens important note though: it does not appear as though the incoming promise is awaited:

node -e "let rs = new ReadableStream(); let it = rs.values(); it.return(new Promise(setTimeout)).then(console.log);"
{ done: true, value: Promise { <pending> } }

Chrome:

Firefox:

Given that information, the best typescript fix to align with existing behavior seems to be:

return { done: true, value: value as undefined };

I've subsequently posted an issue in the WebIDL github to confirm whether this behavior is intentional or an oversight: whatwg/webidl#1522.