allSettled.js

/**
 * Waits for an array of promises to finish, and returns an array of the results.
 *
 * If one or more Promises are rejected, throws a RejectionCollection.
 *
 * This is very similar to `Promise.all`, except it throws ALL errors, instead of just the FIRST error.
 *
 * @param {Array<Promise<T>>} promises
 * @throws {RejectionCollection}
 * @return {Promise<Array<T>>}
 */
function allSettled(promises) {
	return new Promise(function(resolve, reject) {
		var results = new Array(promises.length);
		var remaining = promises.length;
		var errors;

		if (!promises.length)
			return resolve(results);

		promises.forEach(function(promise, index) {
			promise.then(function(result) {
				results[index] = result;
				checkIfAllSettled();
			}, function(error) {
				if (!errors) errors = new RejectionCollection();
				errors.push(error);
				checkIfAllSettled();
			});
		});
		function checkIfAllSettled() {
			if (--remaining) return;
			if (!errors) {
				resolve(results);
			} else {
				reject(errors);
			}
		}
	});
}

function RejectionCollection() { }
RejectionCollection.prototype = Object.assign(new Error(), {
	name: "RejectionCollection",
	push: Array.prototype.push,
	forEach: Array.prototype.forEach,
	toArray: Array.prototype.slice
});


module.exports = allSettled;
module.exports.RejectionCollection = RejectionCollection;

example.js

var allSettled = require("./allSettled.js");

async function doParallelStuff() {
	var async1 = Promise.reject(new Error("Error One"));
	var async2 = Promise.reject(new Error("Error Two"));
	var async3 = Promise.resolve("This one is OK!");
	
	try {
		var results = await allSettled([ async1, async2, async3 ]) 
	} catch (errors) {
		for (let error of errors) {
			console.log(error.message);
		}
		/* Logs:
		 * "Error One"
		 * "Error Two"
		 */
	}
}