function loadjs(url,callback){
    var script=document.createElement(‘script‘);
    script.type="text/javascript";
    script.src=http://www.mamicode.com/url;
    if(script.onreadystatechange){
        script.onreadystatechange=function(){
            if(script.readyState=‘loaded‘||script.readyState=‘complate‘){
                callback();//script is loaded;
            }
        }
    }else{
        script.onloaded=function(){
            callback();//script is loaded;
        }
    }
    document.getElementsByTagName(‘head‘)[0].appendChild(script);
}
function callback(){
alert("script is loaded");
}
//window.onload之后才去加载的js，这样拆分js的加载有利于首页加载速度提高用户体验
window.onload=function(){
    loadjs("jquery.js",callback);
    loadjs("test.js",callback);
}

loadjs异步加载javascript回调