jQuery.getScript加载同域JS

var loadScript = function(url, callback, charset){
var head = document.getElementsByTagName('head')[0];
var script = document.createElement('script');
if(charset){
        script.charset = charset;
    }
    script.src = url;
if(!callback){
return;
    }
    script.Onload= script.Onreadystatechange= function(){
if(!this.readyState || this.readyState==='loaded' || this.readyState==='complete'){
            callback();
        }
    };
    head.appendChild(script);
};