var d=document;

function gebt(s){return d.getElementsByTagName(s)}

function addjs(s){
 var e=d.createElement("script");
 e.src=s+".js";
 e.s=s;
 e.type="text/javascript";
 gebt("head")[0].appendChild(e);
}

addjs("monkey"); will load "monkey.js" file in the same directory.

Dynamically load javascript .js file

JavaScript