How to use setter and getter in javascript, I met a error

Question

My English is not good, but I will try my best to explain my question simply. Description: Alert result is 1, I don't why, I think this should be 2015 to alert.

    var book = {};
    Object.defineProperties(book, {
            _year: {
                value: 1
            },
            edition: {
                value: 23
            },
            year: {
                get: function () {
                    return this._year;
                },
                set: function (newValue) {
                    if (newValue > 2004)
                        this._year = newValue;
                }
            }
        }
    );
    book.year = 2015;
    alert(book.year);

Answer 1

You need to add writable: true like this

_year: {
  value: 1,
  writable: true
},

for __year_.

From Mozilla Developer Network:

writable

true if and only if the value associated with the property may be changed with an assignment operator. Defaults to false.