动态创建 script 标签加载外部 js

    var loadScript = function (url,cb) {
        var script = document.createElement('script');
        script.src = url;
        var firstScript = document.getElementsByTagName('script')[0];
        firstScript.parentNode.insertBefore(script,firstScript);

        // script 加载完毕后调用方法
        script.onload = script.onreadystatechange = function(){
            if( ! this.readyState     //这是FF的判断语句，因为ff下没有readyState这个值，IE的readyState肯定有值
                || this.readyState=='loaded' || this.readyState=='complete'   // 这是IE的判断语句
            ){
                cb && cb()
            }
        };
    }