动态引入js

function loadScript(url, callback){ 

    var script = document.createElement("script") 

    script.type = "text/javascript"; 

    if (script.readyState){ //IE 

        script.onreadystatechange = function(){ 

            if (script.readyState == "loaded" || script.readyState == "complete"){ 

                script.onreadystatechange = null; 

                callback(); 

            }}; 

    }else{

        //Others: Firefox, Safari, Chrome, and Opera 

        script.onload = function(){ 

            callback(); 

        }; 

    } 

    script.src = url; 

    document.body.appendChild(script); 

}