'use strict'; const pLimit = require('p-limit'); class EndError extends Error { constructor(value) { super(); this.value = value; } } // the input can also be a promise, so we `Promise.all()` them both const finder = el => Promise.all(el).then(val => val[1] === true && Promise.reject(new EndError(val[0]))); module.exports = (iterable, tester, opts) => { opts = Object.assign({ concurrency: Infinity, preserveOrder: true }, opts); const limit = pLimit(opts.concurrency); // start all the promises concurrently with optional limit const items = Array.from(iterable).map(el => [el, limit(() => Promise.resolve(el).then(tester))]); // check the promises either serially or concurrently const checkLimit = pLimit(opts.preserveOrder ? 1 : Infinity); return Promise.all(items.map(el => checkLimit(() => finder(el)))) .then(() => {}) .catch(err => err instanceof EndError ? err.value : Promise.reject(err)); };