promises.js

function loadScript(src) {
return new Promise(function(resolve, reject) {
let script = document.createElement('script');
script.src = src;

script.onload = () => resolve(script);
script.onerror = () => reject(new Error("Script load error: " + src));

document.head.append(script);
});
}

loadScript("https://cdnjs.cloudflare.com/ajax/libs/lodash.js/3.2.0/lodash.js")
	.then(script => alert(`${script.src} is loaded!`))
	.catch(error => alert(`Error: ${error.message}`));