(1) defer，只支援IE 

(2) async：

(3) 建立script，插入到DOM中

/**
     * 非同步載入依賴的javascript檔案
     * src：script的路徑
     * callback：當外部的javascript檔案被load的時候，執行的回撥
     */
    function loadAsyncScript(src, callback) {
        var head = document.getElementsByTagName("head")[0];
        var script = document.createElement("script");
        script.setAttribute("type", "text/javascript");
        script.setAttribute("src", src);
        script.setAttribute("async", true);
        script.setAttribute("defer", true);
        head.appendChild(script);
        
        if (script.readyState) {//ie
            script.onreadystatechange = function() {
                var state = this.readyState;
                if (state === 'loaded' || state === 'complete') {
                    callback();
                }
            }
        } else {//Others: Firefox, Safari, Chrome, and Opera 
            script.onload = function() {
                callback();
            }
        }
    }
 

寫法2：

//非同步載入js
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);
}
 

jq寫法：

$.getScript("xxx.js", function(){
       alert("xxx.js載入完畢")
    });