Can I reference other properties during object declaration in JavaScript? [duplicate]

Question

I am trying to do something like this:

var obj = {     a: 5,     b: this.a + 1 } 

(instead of 5 there is a function which I don't want to execute twice that returns a number)

I can rewrite it to assign obj.b later from obj.a, but can I do it right away during declaration?

Answer 1

No. this in JavaScript does not work like you think it does. this in this case refers to the global object.

There are only 3 cases in which the value this gets set:

The Function Case

foo(); 

Here this will refer to the global object.

The Method Case

test.foo();  

In this example this will refer to test.

The Constructor Case

new foo();  

A function call that's preceded by the new keyword acts as a constructor. Inside the function this will refer to a newly created Object.

Everywhere else, this refers to the global object.

Answer 2

There are several ways to accomplish this; this is what I would use:

function Obj() {     this.a = 5;     this.b = this.a + 1;     // return this; // commented out because this happens automatically }  var o = new Obj(); o.b; // === 6