Javascript文件的动态下载

function loadScript(url,callback){
    var script=document.createElement("script");
    script.type="text/javascript";
    if(script.readyState){
	script.onreadystatechange=function(){
	    if(script.readyState=="loaded" || script.readyState=="complete"){
		script.onreadystatechange=null;
		callback();
	    };
	}
    }else{
	script.onload=function(){
	    callback();
    	};
    };
    script.src=url;
    document.getElementsByTagName("head")[0].appendChild(script);
};