Implementations of `flatten` and `flatMap` as described in the [ES Proposal](https://bterlson.github.io/proposal-flatMap/)

flatmap.ts

'use strict';

import { bindCallback } from '../internal/bindcallback';

export async function *flatMapAsync<TSource, TResult>(
    source: AsyncIterable<TSource>,
    fn: (value: TSource) => AsyncIterable<TResult>,
    thisArg?: any): AsyncIterable<TResult> {
  let sel = bindCallback(fn, thisArg, 1);
  for await (let outerItem of source) {
    for await (let innerItem of sel(outerItem)) {
      yield innerItem;
    }
  }
}

flatmapasync.ts

'use strict';

import { bindCallback } from '../internal/bindcallback';

export async function *flatMapAsync<TSource, TResult>(
    source: AsyncIterable<TSource>,
    fn: (value: TSource) => AsyncIterable<TResult>,
    thisArg?: any): AsyncIterable<TResult> {
  let sel = bindCallback(fn, thisArg, 1);
  for await (let outerItem of source) {
    for await (let innerItem of sel(outerItem)) {
      yield innerItem;
    }
  }
}

flatten.ts

'use strict';

import { bindCallback } from '../internal/bindcallback';

export async function *flatMapAsync<TSource, TResult>(
    source: AsyncIterable<TSource>,
    fn: (value: TSource) => AsyncIterable<TResult>,
    thisArg?: any): AsyncIterable<TResult> {
  let sel = bindCallback(fn, thisArg, 1);
  for await (let outerItem of source) {
    for await (let innerItem of sel(outerItem)) {
      yield innerItem;
    }
  }
}

flattenasync.ts

'use strict';

import { isAsyncIterable } from '../internal/isasynciterable';

export async function *flattenAsync<T>(iterable: AsyncIterable<T>, depth: number = Infinity): AsyncIterable<T> {
  if (depth === 0) { return iterable; }
  for await (let item of iterable) {
    if (isAsyncIterable(item)) {
      yield *flattenAsync(item, depth - 1);
    } else {
      yield item;
    }
  }
}