异步加载

<script defer="defer"></script>  <!-- 异步加载（只支持 IE）发生在整个页面解析完毕时 -->

<script src="demo.js" aysnc="aysnc"></script>  <!-- 异步加载文件（W3C 官方语法）只能用于加载外部脚本 -->

<script>
    var script = document.createElement('script');  // 创建
    script.type = "text/javascipt"; // 设置
    script.src = "demo.js";  // 发送请求 下载
    script.onload = function(){  // IE script 标签上没有 load 事件
        text();
    }
    document.head.appendChild(script);
</script>

<!-- 封装 loadScript 事件 -->
<script>
    function loadScript(url, callback){
        var script = document.createElement('script');
        script.type = "text/javascript";
        if(script.readyState){  // 兼容 IE
            script.onreadystatechange = function(){
                if(script.readyState == "complete" || script.readyState == "onaded"){
                    callback();
                }
            }
        }else{
            script.onload = function(){
                callback();
            }
        }
        script.src = url;
        document.head.appendChild(script);
    }
    loadScript('demo.js', function(){  // 函数调用
        text();
    });
</script>