Load any Script files - The asynchronous way

snippet.js

/* Example of how to load Facebook SDK */
loadAsync(document, 'script', '//connect.facebook.net/en_US/sdk.js', 'facebook-jssdk', successCallback);

function successCallback () {
  console.log('Facebook\'s SDK successfully loaded.');
}

/* Asynchronously load any script files */
function loadAsync ( doc, elem, source, id, cb ) {
	var ready = false,
	    newElem,
	    tag = doc.getElementsByTagName(elem)[0];

	if ( doc.getElementById(id) ) return;

	newElem = doc.createElement(elem);
	newElem.id = id;
	newElem.async = 'async';
	newElem.src = source;

	if ( typeof(cb) === "function" ) {
		newElem.onload = newElem.onreadystatechange = function () {
			if ( !ready && ( !this.readyState || this.readyState == 'complete' ) ) {
				ready = true;
				cb();
			}
		};
	}

	tag.parentNode.insertBefore(newElem, tag);
}