异步加载src

function add(url,cellback){                
        var script=document.createElement("script");        
        if (script.readyState) {
            console.log(script.readyState)
             script.onreadystatechange = function  () {                 
                 if(script.readyState == "complete" || script.readyState == "loaded"){
                     cellback();                                          
                 }
             }              
          }else{
              script.onload = function(){
                  cellback();
              }
          }    
           script.src = url;
           
            document.head.appendChild(script)
    }    
    add("620.ex.js",function(){
        test()
    })