Question

Hello I found some difficulties when implementing javascript Object.defineProperties:

var book1 = {};
Object.defineProperties(book1, {
_year: {
    value: 2004
},
edition: {
    value: 1
},
year: {
    get : function() {
        return this._year;
    },
    set : function(newValue) {
        if ((newValue - this._year) > 0) {
            this.edition += 1;
        } else if ((newValue - this._year) < 0) {
            this.edition -= 1;
        }
        this._year = newValue;
    }
}
});
book1.year = 2005;
document.write(book1.edition);  //get 1, expect 2
document.write('<br/>');
book1.year = 2006;
document.write(book1.edition); //get 1, expect 3
document.write('<br/>');

Browser: Chrome 17.0.963.56

Any answer is welcome.
Thank you.

Answer 1

You have to specify writable: true as a property descriptor of _year. By default, it’s not writable, and assigning a value to a non-writable property doesn’t have any effect.

I strongly recommend to activate the strict mode, because you will receive an error message when the assignment of a value to a read-only property fails.

"use strict"; // <---
Object.defineProperties(book1, {
    _year: {
        value: 2004
        writable: true  /* <-- writable set to true*/
    },
    edition: {
        value: 1,
        writable: true
    },