如何判断js是否加载完全

var script=document.createElement('script');
  		if(script.onreadystatechange){
  			script.onreadystatechange=function(){
  			if(this.readyState==='loaded' || this.readyState==='complete'){
  				script.onreadystatechange=null;
  				callback();
  			}
  			}
  		}else{
  			script.onload=function(){
  				callback();
  			}
  		}
  		script.setAttribute("src",url);
  		document.body.appendChild(script);