Promise-based Asynchronous Script Loader

loadScript.js

/**
 * Loads javascript file by url and saves reference to it by name, so it's not loaded again
 * @param  {string} name Name of js library
 * @param  {string} url  Url to js library
 * @return {promise}     Resolves when library is loaded
 * 
 * Based on example by Brad Berger: https://bradb.net/blog/promise-based-js-script-loader/
 * Extended w/ global variable to keep track of previously loaded scripts.
 * Removed support for loading several independent scripts at once via Promise.all()
 * 
 * If you load a js file (e.g. jQuery) w/out this loader, be sure to append this at the end of it:
 * gc.scripts.jquery = Promise.resolve();
 */
gc.scripts = {};
gc.loadScript = function(name, url)  {

	// script has already been loaded (or is in-flight)
	if (gc.scripts.hasOwnProperty(name)) {
		return gc.scripts[name];
	}

	// promise resolves when script has been loaded
	var p = new Promise(function (resolve, reject) {
		var r = false,
			h = document.getElementsByTagName("head")[0],
			s = document.createElement("script");

		s.src = url;
		s.async = true;
		s.onload = s.onreadystatechange = function () {
			if (!r && (!this.readyState || this.readyState == "complete")) {
				r = true;
				resolve(this);
			}
		};
		s.onerror = s.onabort = reject;
		h.appendChild(s);
	});

	// store record of promise so js file is never loaded twice
	gc.scripts[name] = p;

	// return promise so it can be used to trigger callback function
	return p;
}