Load script files with a function call.

loadES.js

var nil = 1;

function getFirst(elm) {
	var firstElm = document.getElementsByTagName(elm)[0];

    if (firstElm == undefined)
        return nil;

    return firstElm;
}

function loadES(url) {
	var nscriptElm = document.createElement('script');
	var request = new XMLHttpRequest();

	try {
		request.overrideMimeType('text/plain');
		request.open("GET", url, false);
		request.send();
	} catch(error) {
		return nil;
	}
	
	nscriptElm.appendChild(
		document.createTextNode(request.response));
   
	getFirst('head').appendChild(nscriptElm);
}