Function mergeAsyncIterables

packages/utils/src/internals/iterables.ts:249–281

(...iterables: AsyncIterable<T>[])

 * ```
 */
export async function* mergeAsyncIterables<T>(...iterables: AsyncIterable<T>[]): AsyncIterable<T> {
    const asyncIterators = iterables.map((iterable) => iterable[Symbol.asyncIterator]());
    const results = [];
    let count = asyncIterators.length;
    const never: Promise<never> = new Promise(() => {});
    async function getNext(asyncIterator: AsyncIterator<T>, index: number) {
        const result = await asyncIterator.next();
        return {
            index,
            result,
        };
    }
    const nextPromises = asyncIterators.map(getNext);
    try {
        while (count) {
            const { index, result } = await Promise.race(nextPromises);
            if (result.done) {
                nextPromises[index] = never;
                results[index] = result.value;
                count--;
            } else {
                nextPromises[index] = getNext(asyncIterators[index], index);
                yield result.value;
            }
        }
    } finally {
        for (const [index, iterator] of asyncIterators.entries()) {
            // no await here - see https://github.com/tc39/proposal-async-iteration/issues/126
            if (nextPromises[index] !== never && iterator.return != null) void iterator.return();
        }
    }
    return results;
}