function loadJS(url, callback) {
            var head = window.document.getElementsByTagName("head")[0];
            var script = window.document.createElement('script');
            script.onload = script.onreadystatechange = script.onerror = function() {
                if (script && script.readyState && /^(?!(?:loaded|complete)$)/.test(script.readyState)) return;
                script.onload = script.onreadystatechange = script.onerror = null;
                script.src = '';
                script.parentNode.removeChild(script);
                script = null;
                if (callback) {
                    callback();
                }
            };
            script.charset = "utf-8";
            script.async = true;
            script.src = url;
            try {
                head.appendChild(script);
            } catch (exp) {}
        }

        function loadJS2(url,callback){
            var head=document.getElementsByTagName('head')[0];
            var script=document.createElement('script');

            if(script.readyState){
                script.onreadystatechange=function(){
                    if(script.readyState=='loaded' || script.readyState=='complete'){
                        script.parentNode.removeChild(script);

                        callback && callback();
                    }
                };
            }else{
                script.onload=function(){
                    script.parentNode.removeChild(script);

                    callback && callback();
                };
            }

            script.src=url;
            head.appendChild(script);
        }

        console.time('start');
        for(var i=0; i<100; i++){
            loadJS2('https://code.jquery.com/jquery-3.2.1.js',function(){
                console.log('count:'+i);
            });
        }
        console.timeEnd('start');