// utility for whatwg streams

// The living standard of whatwg streams says

// ReadableStream is also AsyncIterable, but

// as of June 2019, no browser implements it.

// See https://streams.spec.whatwg.org/ for details

export type ReadableStreamLike<T> = AsyncIterable<T> | ReadableStream<T>;

export function isAsyncIterable<T>(object: ReadableStreamLike<T>): object is AsyncIterable<T> {

return (object as any)[Symbol.asyncIterator] != null;

}

function assertNonNull<T>(value: T | null | undefined): asserts value is T {

if (value == null) {

throw new Error("Assertion Failure: value must not be null nor undefined");

}

}

export async function* asyncIterableFromStream<T>(stream: ReadableStream<T>): AsyncIterable<T> {

const reader = stream.getReader();

try {

while (true) {

const { done, value } = await reader.read();

if (done) {

return;

}

assertNonNull(value);

yield value;

}

} finally {

reader.releaseLock();

}

}

export function ensureAsyncIterabe<T>(streamLike: ReadableStreamLike<T>): AsyncIterable<T> {

if (isAsyncIterable(streamLike)) {

return streamLike;

} else {

return asyncIterableFromStream(streamLike);

}

}