Very simple interface for loading resources (esp. images, audio for e.g. game sprites) asynchronously, attaching callbacks to individual resources, and waiting for all queued resources to be loaded.

Loader.js

"use strict";

/* Simple async resource loader */

class Loader {
	constructor() {
		this.promises = [];
	}

	get(src, type = Image) {
		let promise = new Promise(function (fulfill, reject) {
			let res = new type;

			res.onload = function () {
				fulfill(res);
			};

			res.onerror = function () {
				reject();
			};

			res.src = src;
		});

		this.promises.push(promise);

		return promise;
	}

	// Resolves when all queued resources have finished loading
	// IMPORTANT: To release memory and to clear promises for failed downloads,
	// wait() immediately clears the queue as a side-effect
	wait() {
		let promise = Promise.all(this.promises);
		this.promises = [];
		return promise;
	}
}