`async.mapLimit` as a `promise`

index.js

const Promise = require('bluebird');

/**
 * Produce a new collection of values by asynchronously mapping each value in
 * `collection` through the `iteratee` function, but running a maximum of
 * `limit` operations at a time.
 *
 * __Inspired by `async.mapLimit`__
 *
 * @param {Array} collection
 * @param {Integer} limit
 * @param {Function} iteratee
 * @return {Promise}
 */
function mapLimit(collection, limit, iteratee) {
	let queued = [];
	const promises = collection.map((item) => {
		const someLimit = Math.max(0, queued.length - limit + 1);
		const iterate = Promise.some(queued, someLimit)
			.then(() => {
				return iteratee(item);
			});
		queued.push(iterate);
		return iterate;
	});
	return Promise.all(promises);
}