Cannot access the variable in the object

Question

I want to access the variable in the object, something like below:

var menuSetup = {
    m : [100,200],
    height: m[0], // m is not defined
}


var menuSetup = {
    m : [100,200],
    height: this.m[0], // I tried this, still failed
}

So can I access the variable in the object?

You could define a variable outside of the object and use that in both places. http://jsfiddle.net/5taBt/ — Patrick Q, Jul 17 '14 at 18:49
Why would you want to do this? It would cause cluttering. What you really want here is an object with a *method* `height()` that returned the value you want. — Casey Falk, Jul 17 '14 at 18:52
Sorry, let me rephrase: why would you *ever* want to do this? This is *begging* for trouble since JavaScript is reference-based. — Casey Falk, Jul 17 '14 at 19:06
@Casey Falk Sorry, I am just a newbie. Yes, I know the problem, thanks! — CSnerd, Jul 17 '14 at 19:09
No worries! It's just best to be aware of dangers early on so you don't fall down the rabbit hole. ;) It's good to be curious. — Casey Falk, Jul 17 '14 at 19:12

score 0 · Accepted Answer · answered Jul 17 '14 at 18:54

Unfortunately, you can't do that. This is because the properties of menuSetup haven't been defined yet. For example,

var menuSetup = {
  m: [100, 200],
  height: menuSetup.m[0]
};

will through (in chrome) TypeError: Cannot read property 'm' of undefined because menuSetup is just undefined (because it was hoisted by the var declaration)

The only two ways I can think of is to

a) Save m as a variable before. For example:

var m = [100, 200];
var menuSetup = {
  m: m,
  height: m[0]
}

b) Use a method instead of a variable and execute it.

var menuSetup = {
  m: [100, 200],
  height: function() { return this.m[0]; }
}

Then, when you get menuSetup.height, you would actually do menuSetup.height().

score 0 · Answer 2 · answered Jul 17 '14 at 18:54

0

You can try this:

var MenuSetup = function() {
    this.m =  [100,200];
    this.height = this.m[0];
}

var menuSetup = new MenuSetup();
console.log(MenuSetup.height);

answered Jul 17 '14 at 18:54

Yuriy Galanter

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I always get beat out by about 15 seconds :P – mc01 Jul 17 '14 at 18:55
1

Shouldn't the constructor start with a capital letter and the instance start with the lower i.e. `var menuSetup = new MenuSetup()`? – soktinpk Jul 17 '14 at 18:56
@soktinpk good point, I was keeping OP casing but you're right this is different and merit changing. – Yuriy Galanter Jul 17 '14 at 18:57

score 0 · Answer 3 · edited May 23 '17 at 12:21

Not the way you're doing it. In JavaScript you cannot reference another property while creating the same object, because the object doesn't yet exist until after the closing } at the end.

You could use a constructor function instead though, as explained in answers to this similar question.

function MenuSetup() {
    this.m = [100, 200];
    this.height = this.m[0];
}

var menu = new MenuSetup();
console.log(menu.height) //100

Cannot access the variable in the object

3 Answers3