index.js

'use strict';
module.exports = (iterable, mapper, opts) => new Promise((resolve, reject) => {
	opts = Object.assign({
		concurrency: Infinity
	}, opts);

	if (typeof mapper !== 'function') {
		throw new TypeError('Mapper function is required');
	}

	const concurrency = opts.concurrency;

	if (!(typeof concurrency === 'number' && concurrency >= 1)) {
		throw new TypeError(`Expected \`concurrency\` to be a number from 1 and up, got \`${concurrency}\` (${typeof concurrency})`);
	}

	const ret = [];
	const iterator = iterable[Symbol.iterator]();
	let isRejected = false;
	let iterableDone = false;
	let resolvingCount = 0;
	let currentIdx = 0;

	const next = () => {
		if (isRejected) {
			return;
		}

		const nextItem = iterator.next();
		const i = currentIdx;
		currentIdx++;

		if (nextItem.done) {
			iterableDone = true;

			if (resolvingCount === 0) {
				resolve(ret);
			}

			return;
		}

		resolvingCount++;

		Promise.resolve(nextItem.value)
			.then(el => mapper(el, i))
			.then(
				val => {
					ret[i] = val;
					resolvingCount--;
					next();
				},
				err => {
					isRejected = true;
					reject(err);
				}
			);
	};

	for (let i = 0; i < concurrency; i++) {
		next();

		if (iterableDone) {
			break;
		}
	}
});