动态添加脚本，并触发回调函数 初步实现按需加载

 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
        script.onload = function(){
            callback();
        };
    }
    script.src = url;
    document.getElementsByTagName("head")[0].appendChild(script);
}

调用方法：

loadScript("http://ajax.googleapis.com/ajax/libs/jquery/1.2.6/jquery.min.js", function(){
	alert('load success1!');
});