How to create instance variables in JavaScript?

Question

My understanding is that things on the prototype chain are only created once per object and that to create instance variables, one must use this. Is this correct?

For example:

function Foo () {
    this.some_instance = "hello";
}
Foo.prototype.some_static = "hi";

Implementation

var foo1 = new Foo();

var foo2 = new Foo();

Making a foo1 and a foo2 will create two values of hello but only one of hi.

Is this correct?

Yes the some_static variable will not be recreated for each instance and will exist on the prototype chain — megawac
– megawac, Commented Nov 3, 2013 at 1:23
the bigger question, was how to create instance variables with out using this, is it possible? I have a framework that create "classes" that I'm trying to update. — employee-0
– employee-0, Commented Nov 3, 2013 at 1:25

Broxzier · Accepted Answer · 2013-11-03 01:29:50Z

3

Short answer: Yes.

Proof:

function Foo () {
    this.some_instance = "hello";
}

var foo1 = new Foo();
Foo.prototype.some_static = "hi"; // in between
var foo2 = new Foo();

foo1.some_static === foo2.some_static; // return true

answered Nov 3, 2013 at 1:29

Broxzier

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2 Comments

megawac Over a year ago

Thats a poor test a better test would be Foo.prototype.some_static = {a:1}

HMR Over a year ago

I agree with megawac, that only proves that "hi"==="hi" you can define some_static as this.some_static, assign "hi" to it and the === will still be true. As megawac pointed out you can notice the sharing when you have mutable objects as your property value because assigning values to a property will cause JS to not look up on the prototype chain. Instead it'll just create that property on the instance and assign the value to it. foo1.some_static=... will create some_static on foo1 instance. stackoverflow.com/a/16063711/1641941

JW. · Accepted Answer · 2013-11-03 01:31:04Z

Well, making a foo1 and a foo2 won't create any values of "hi". The "some_static" variable with the value "hi" was already created, in the Foo prototype object, before you created foo1 and foo2.

When you create objects that use the Foo prototype, and you reference their "some_static" property, it will look in the objects themselves first. If the objects don't have a "some_static" property, it will look for it in their prototype object.

But yes, if you want to create a property that's unique to each object that uses the same prototype, you should set it on the object itself, not the prototype. You can do that by setting "this.some_instance" from inside one of the object's methods, or by setting "foo1.some_instance" from outside.

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