实现动态加载一个 JavaScript 资源

var script = document.createElement("script");
        var head = document.getElementsByTagName("head")[0];

        script.type = "text / javascript";
        script.src = " //i.alicdn.com/resource.js";

            // 绑定资源加载成功事件
            script.onreadystatechange = function () {
                // 判断资源加载状态是否为加载成功或加载完成
                if (/^(loaded|complete)$/.test(script.readyState)) {
                    script.onreadystatechange = null;
                    
                }
            };

        // 绑定资源加载失败事件
        script.onerror = function () {
            
        };

        head.insertBefore(script, head.firstChild)
    </script>