Multiple Object Properties JavaScript

Question

Got this code snippet in JavaScript which defines multiple properties into an object.

var book = {};

Object.defineProperties(book , {
    _year: {
        value: 2004
    },

    edition: {
        value: 1
    },

    year: {
        get: function() {
            return this._year;
        },

        set: function(newValue) {
            if (newValue > 2004) {
                this._year = newValue;
                this.edition += newValue - 2004;
            }
        }
    }
});

book.year = 2005;
alert(book.edition);

So the book which has that code claims that alert(book.edition);would display 2.Instead , it displays 1. Seems like it never executes the part of the accessors property code (year: get: set:). Anyone knows why this is happening?

Fabrício Matté · Accepted Answer · 2014-06-01 04:30:46Z

6

edition has been defined as a non-writable property -- the writable descriptor property hasn't been specified and is false by default. Hence this.edition += ... fails silently. The same applies to the _year property as well.

_year: {
    writable: true,
    value: 2004
},
edition: {
    writable: true,
    value: 1
},

Fiddle

Note: assigning to a non-writable property throws an error in strict mode, so you can add the 'use strict'; pragma to spot these errors more easily.

Reference:

MDN - Object.defineProperties()
MDN - Object.defineProperty() -- explains descriptors more in-depth.

edited Jun 1, 2014 at 4:30

answered Jun 1, 2014 at 3:48

Fabrício Matté

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6 Comments

Roko C. Buljan Over a year ago

I would ask a refound! +1

user3649693 Over a year ago

Should i add 'use strict;' on top of my script , just above var book = {}; or above Object.defineProperties(book , { , to test this error in strict mode? i noticed that script throws an error if i put 'use strict;' above var book = {}; , but if i place it above Object.defineProperties(book , { it still executes , gives a result of 1 and no error is thrown. On the other hand , if i place it just below Object.defineProperties(book , { script never executes even if i put writable: true on both properties (_year and edition).This is really confusing

Fabrício Matté Over a year ago

@user3649693 the 'use strict'; pragma must be the first statement in a script (to apply to the entire script) or the first statement in a function (to apply to that individual function) -- see the Invoking strict mode section for details. Also, don't enable it for an entire script file if that file will later be concatenated with other script files, prefer wrapping the script's code into an IIFE with strict mode enabled in that case.

Fabrício Matté Over a year ago

@user3649693 and in case you're not familiar with IIFEs, see this article by Ben Alman.

Fabrício Matté Over a year ago

@user3649693 exactly. And if you put it on top of a script, then strict mode is enabled for the entire script, including all of its functions -- specifying it per-function is redundant then as strict mode is already enabled for the entire script.

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