Function once

vue.js:481–489

* Ensure a function is called only once. * 确保该函数只调用一次 闭包函数

(fn)

     *  确保该函数只调用一次 闭包函数
     */
    function once(fn) {
        var called = false;
        return function () {
            if (!called) {
                called = true;
                fn.apply(this, arguments);
            }
        }
    }

    //ssr标记属性
    var SSR_ATTR = 'data-server-rendered';