How to have one member of a javascript object refer to another member of same object?

Question

While defining an object in javascript, is there a way for a member of that object to access the value of another member of the same object? For example if I am defining the below (using jquery)

var x = $.extend({}, {
  greeting: 'hello',
  message: this.greeting+' world'
});

alert(x.message);

This above code would alert undefined world. How do I make it take the value of greeting during the definition. In this case the expected output is hello world.

Related: http://stackoverflow.com/questions/133973/how-does-this-keyword-work-within-a-javascript-object-literal — m90, Apr 18 '13 at 18:32
`this` in that case is the window, not the object, and since the object isn't defined anywhere yet, there isn't anything to reference yet. So, no, you can't do that. — Kevin B, Apr 18 '13 at 18:33

score 8 · Answer 1 · answered Apr 18 '13 at 18:35

8

You can use properties:

var foo = {
    greeting: 'hello',
    get message() {
        return this.greeting+' world'
    }
};

console.log(foo.message);
foo.greeting = 'hi';
console.log(foo.message);

Although I'm not sure how these will interact with $.extend

In older browsers, you could always just make message a function, rather than a property getter

answered Apr 18 '13 at 18:35

Eric

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This is a nice way to do it, but it won't work in old browsers. – Pointy Apr 18 '13 at 18:35
Wow, how long have these been available? – Snuffleupagus Apr 18 '13 at 18:36

score 2 · Answer 2 · answered Apr 18 '13 at 18:36

2

Try this way:

var x = $.extend({}, {
  greeting: 'hello',
  message: function(){
     return this.greeting+' world';
  }
});

alert(x.message);

answered Apr 18 '13 at 18:36

Marcelo Rodovalho

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Sorry, the @Eric answered first! – Marcelo Rodovalho Apr 18 '13 at 18:37
2

You're not invoking `x.message`, you should use `console.log` over `alert` for debugging. – Paul S. Apr 18 '13 at 18:37
Oh yeah, you are right, I just paste the same code with the modification. – Marcelo Rodovalho Apr 18 '13 at 18:38
2

This should read `alert(x.message());` – Eric Apr 18 '13 at 18:38

score 0 · Answer 3 · answered Apr 18 '13 at 18:40

0

You can create some Constructor function. Inside this function this.greeting will be defined.

var Constructor = function () {
   this.greeting = 'hello',
   this.message = this.greeting + ' world'
   return this;
};

And then extend your object with instance of Constructor:

var x = $.extend({}, new Constructor());

answered Apr 18 '13 at 18:40

Artyom Neustroev

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You are right too, but in this case he is using a literal object. – Marcelo Rodovalho Apr 18 '13 at 18:43
I guess, it's just a code snippet. I think, that this method is a good choice, if having a literal object is not mandatory. – Artyom Neustroev Apr 18 '13 at 18:49
1

There's no need to `return this` here – Eric Apr 18 '13 at 23:20
@Eric Really! The scope of the function already does this by default – Marcelo Rodovalho Apr 19 '13 at 13:30

How to have one member of a javascript object refer to another member of same object?

3 Answers3