jQuery.getScript加载同域JS的代码

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); 
};