mapasync.ts

'use strict';

class MapAsyncIterable<T, R> implements AsyncIterable<R> {
    private _source: Iterable<T | PromiseLike<T>> | AsyncIterable<T>;
    private _selector: (value: T) => R | PromiseLike<R>;

    constructor(source: Iterable<T | PromiseLike<T>> | AsyncIterable<T>, selector: (value: T) => R | PromiseLike<R>) {
        this._source = source;
        this._selector = selector;
    }

    async *[Symbol.asyncIterator]() {
        for await (let item of this._source as AsyncIterable<T>) { // workaround for https://github.com/Microsoft/TypeScript/issues/16153
            let result = await this._selector(item);
            yield result;
        }
    }
}


export function mapAsync<T, R>(
    source: Iterable<T | PromiseLike<T>> | AsyncIterable<T>,
    selector: (this: undefined, value: T) => R | PromiseLike<R>,
    thisArg?: undefined | null): AsyncIterable<R>;

export function mapAsync<T, R, This>(
    source: Iterable<T | PromiseLike<T>> | AsyncIterable<T>,
    selector: (this: This, value: T) => R | PromiseLike<R>,
    thisArg: This): AsyncIterable<R>;

export function mapAsync<T, R>(
    source: Iterable<T | PromiseLike<T>> | AsyncIterable<T>,
    selector: (value: T) => R | PromiseLike<R>,
    thisArg?: any): AsyncIterable<R> {
    const fn = selector.bind(thisArg);
    return new MapAsyncIterable<T, R>(source, fn);
}