IllusionTurtle

https://jsfiddle.net/vu14Lcst/

ванильно
https://jsfiddle.net/soq2bxbz/1/

function include(url,callBack){
var script = document.createElement('script');
script.src=window.location.protocol+'//'+window.location.host+url;
document.getElementsByTagName('head')[0].appendChild(script);
script.onload = function(){window[callBack]};
}
include('/path/to/js/script.js','includeFinished')