13 实现单例模式

核心要点: 用闭包和Proxy属性拦截

    function proxy(func) {
        let instance;
        let handler = {
            constructor(target, args) {
                if(!instance) {
                    instance = Reflect.constructor(fun, args);
                }
                return instance;
            }
        }
        return new Proxy(func, handler);
    }