Object referencing its own property on initialization [duplicate]

Question

Possible Duplicate:
Self-references in object literals / initializers

Can this be done? (obviously not in this syntax)

var a = {
    b : 10,
    c : this.b * 2 // returns 'undefined'
};

I have also tried

var a = {
    b : 10,
    c : a.b * 2 // throws error 'a is undefined'
};

and

var a = {
    b : 10,
    c : b * 2 // throws error 'b is undefined'
};

It makes sense to me that these values are undefined, I have not finished defining them. However it seems to me like there would be a solution to structuring a object like that and having c be conditional on b

Considering you are initializing b at compile time, why would c be conditional on it? You already know what it is! — Chad La Guardia
– Chad La Guardia, Commented Sep 15, 2011 at 15:30
What I have posted is a simplified example. I will not always know what b is. Otherwise I wouldn't need to have it conditional. think b : function(x) { /* do some stuff */ return y;}, — rlemon
– rlemon, Commented Sep 15, 2011 at 15:32
@Chad 1. JS isn't compiled, 2. it's perfectly normal to initialise a variable with an expression based on another, and other languages (e.g. C) manage it perfectly well. — Alnitak
– Alnitak, Commented Sep 15, 2011 at 15:32
@Alnitak compiled is the wrong word for JS... but my point is the same. He is hardcoding a value (equivalent to knowing something at compile time). However, the OP has clarified that he may not actually know the value, so the point is moot anyway. — Chad La Guardia
– Chad La Guardia, Commented Sep 15, 2011 at 15:33
thankyou @Felix Kling! I couldn't find any posts on this when I first searched it. I like the getters solution. +1 — rlemon
– rlemon, Commented Sep 15, 2011 at 15:35

Skylar Anderson · Accepted Answer · 2011-09-15 15:40:13Z

15

Alternatively, you can use a self starting function to give you a similar affect to what you are looking for:

var a = (function() {
    var b = 10;
    return {
        b:b,
        c:b*2
    }
})();

console.log(a.c);

answered Sep 15, 2011 at 15:40

Skylar Anderson

5,7131 gold badge27 silver badges34 bronze badges

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

2 Comments

rlemon Over a year ago

I like that, seems almost hacky

vol7ron Over a year ago

@rlemon: if you like that, why don't you just do it in two parts to void yourself of the function overhead? In fact, this is clutter. It is still doing everything in two steps and on top of that, you're calling an unnecessary function.

Dennis · Accepted Answer · 2011-09-15 15:33:19Z

1

Not using object literal notation, but you can use functions to create it:

function Structure(value) {
    this.b = value;
    this.c = 2 * value;
}

var a = new Structure(10);

or

function makeStructure(value) {
    return {b:value, c:2*value};
}

var a = makeStructure(10);

answered Sep 15, 2011 at 15:33

Dennis

32.7k12 gold badges66 silver badges79 bronze badges

Collectives™ on Stack Overflow

Object referencing its own property on initialization [duplicate]

2 Answers 2

2 Comments

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Linked

Related