Scope chains in object literals

Question

var x = 9;
var mod = {
    x: 81,
    assign: function(){
        this.x = 9;
        x = 3;
    },
    checkVars: function(){
        alert(x + " - " + this.x );
    }
};

mod.checkVars(); //9 - 81

mod.assign();
mod.checkVars(); //3 - 9

alert(x); //3

Please explain how does scope chains set themselves here. And why does scope resolution for x in checkVars and assign skip the object mod?

`mod` doesn't have a variable called `x`. It has a *property* called `x`. — Mike Christensen, Jun 21 '13 at 17:26
I never said `x` is a variable. `checkVars` is just a fancy name, if you meant it. — Max Ch, Jun 21 '13 at 17:49
What you seem to be missing is that scope chains only apply to variables, not properties (the prototype chain applies to those). — bfavaretto, Jun 21 '13 at 17:50

Mike Christensen · Answer 1 · 2013-06-21T17:51:04.600

5

I've added some comments to your program:

var x = 9; // This is the *only* variable called x in your program
var mod = {
    x: 81, // this x refers to a property of mod also called x
    assign: function(){
        this.x = 9; // "this" refers to the object mod, this.x is the x property of mod
        x = 3; // x here refers to your variable called x
    },
    checkVars: function(){
        alert(x + " - " + this.x ); // same as above
    }
};

mod.checkVars(); //9 - 81

mod.assign();
mod.checkVars(); //3 - 9

alert(x); //3

In other words, your confusion doesn't have anything to do with scope resolution. Any time you reference x, you're referring to the one and only variable called x you defined at the top of your program. Any time you refer to this.x, you're referring to the property called x you defined on the mod object literal.

Hope this helps clarify things!

edited Jun 21 '13 at 17:51

answered Jun 21 '13 at 17:32

Mike Christensen

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`this x refers to your variable called x` maybe you should change it to: `x here refers to your variable called x` - Just to eliminate any misunderstandings about "this x" and `this.x` – Andreas Louv Jun 21 '13 at 17:43
@NULL - Heh, yes that might be less confusing. – Mike Christensen Jun 21 '13 at 17:44
Actually there is *some* scope resolution going on. There is no local `x` inside the functions, and `x` is found in the parent level of the scope chain. – bfavaretto Jun 21 '13 at 17:48
@bfavaretto - Eh, I figured people would know what I meant. But I've changed the wording. – Mike Christensen Jun 21 '13 at 17:52
Yeah, sometimes I'm too picky and don't even notice it... Sorry about that. – bfavaretto Jun 21 '13 at 17:53
Simply speaking within an object literal , to access its properties in your case x of mod, you have to use "this" to define context – jason Jun 21 '13 at 17:57

score 1 · Accepted Answer · answered Jun 21 '13 at 17:36

Variables and object properties are not the same thing. Variables are part of scope chains, properties are not. The only variables in scope of assign and checkVars are x and mod. The properties of mod are only visible from within those methods via this.propName (or mod.propName).

Scope chains in object literals

2 Answers2