index.js

'use strict';
const AggregateError = require('aggregate-error');

module.exports = async (
	iterable,
	mapper,
	{
		concurrency = Infinity,
		stopOnError = true
	} = {}
) => {
	return new Promise((resolve, reject) => {
		if (typeof mapper !== 'function') {
			throw new TypeError('Mapper function is required');
		}

		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 errors = [];
		const iterator = iterable[Symbol.iterator]();
		let isRejected = false;
		let isIterableDone = false;
		let resolvingCount = 0;
		let currentIndex = 0;

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

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

			if (nextItem.done) {
				isIterableDone = true;

				if (resolvingCount === 0) {
					if (!stopOnError && errors.length !== 0) {
						reject(new AggregateError(errors));
					} else {
						resolve(ret);
					}
				}

				return;
			}

			resolvingCount++;

			(async () => {
				try {
					const element = await nextItem.value;
					ret[i] = await mapper(element, i);
					resolvingCount--;
					next();
				} catch (error) {
					if (stopOnError) {
						isRejected = true;
						reject(error);
					} else {
						errors.push(error);
						resolvingCount--;
						next();
					}
				}
			})();
		};

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

			if (isIterableDone) {
				break;
			}
		}
	});
};