【红宝书p207】访问器属性用法

let book = {
            // 对象里面的属性实则可以被直接访问
            year_: 2017,
            edition: 1
        };

准备以上对象，通过访问器属性间接修改对象里面属性的值
es5的用法：

 // es5
        Object.defineProperty(book, "year", {
            get() {
                return this.year_;
            },
            set(newValue) {
                if (newValue > 2017) {
                    this.year_ = newValue;
                    this.edition += newValue - 2017;
                }
            }
        });
        console.log(book.year_, book.edition);//2017 1
        book.year = 2018;
        console.log(book.year_, book.edition);//2018 2

es5之前的用法

 book.__defineGetter__("year", function () {
            return this.year_;
        });
        book.__defineSetter__("year", function (newValue) {
            this.year_ = newValue;
            this.edition += newValue - 2017;
        });
        console.log(book.year_, book.edition);//2017 1
        book.year = 2018;
        console.log(book.year_, book.edition);//2018 2