Question about Javascript pointer to an object property

Question

I wanted to set a variable to point to property in an newly created object to save a "lookup" as shown in the example below. Basically, I thought the variable is a reference to the object's property. This is not the case; it looks like the variable holds the value. The first console.log is 1 (which is the value I want to assign to photoGalleryMod.slide) but when looking at photoGalleryMod.slide, it's still 0.

Is there a way to do this? Thanks.

(function() {
    var instance;

    PhotoGalleryModule = function PhotoGalleryModule() {

        if (instance) {
            return instance;
        }

        instance = this;

        /* Properties */
        this.slide = 0;
    };
}());

window.photoGalleryMod = new PhotoGalleryModule();

/* Tried to set a variable so I could use test, instead of writing photoGalleryMod.slide all the time plus it saves a lookup */

var test = photoGalleryMod.slide;

test = test + 1;

console.log(test);
console.log(photoGalleryMod.slide);

Pointy · Accepted Answer · 2011-04-14 18:30:42Z

3

Yes, you're making a copy of the value, because "slide" is set to a primitive type. Try this:

this.slide = [0];

and then

var test = photoGalleryMod.slide;
test[0] = test[0] + 1;

then change the logging:

console.log(test[0]);
console.log(photoGalleryMod.slide[0]);

In that case you'll see that you do have a reference to the same object. Unlike some other languages (C++) there's no way to say, "Please give me an alias for this variable" in JavaScript.

answered Apr 14, 2011 at 18:30

Pointy

415k62 gold badges600 silver badges631 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Stephen Over a year ago

That works. Unfortunately, slide is just a primitive, so T.J. Crowder said I already have it right. I can't make this code any shorter.

Pointy Over a year ago

Yes, that's true - I just mean what I wrote as an illustrative example.

T.J. Crowder · Accepted Answer · 2011-04-14 18:29:40Z

2

it looks like the variable holds the value

That's correct. Since you're using number primitives, variables contain the value rather than pointing to it. Variables only contain references when they're referring to objects.

The way to do it is to use an object property and to point to the object — which is exactly what you have with photoGalleryMod and its slide property.

answered Apr 14, 2011 at 18:29

T.J. Crowder

1.1m201 gold badges2k silver badges2k bronze badges

1 Comment

Stephen Over a year ago

Thanks for the confirmation. Just wanted to make sure I did not misunderstanding this aspect of Javascript.

Collectives™ on Stack Overflow

Question about Javascript pointer to an object property

2 Answers 2

2 Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related