How to reference an object inside itself?

Question

As you can see below I'm creating an unnamed object inside the console.log

    console.log({
      x: 2,
      y: this.x * 2,
    });

I'm not assigning it to any variable.

So I want a way to access it's property x inside itself like calling this.x.

However the above approach is not working.

I'm getting a NAN error.

NOTE I don't want it to assign to a variable/ create using a function constructor/ prototype.

If I just use the variable then it throws x not defined error... Hope you got my point. — mx_code, Oct 08 '20 at 14:11
I just want to access a property/variable of an object within itself... something like we do with a class ... like accessing own props using this keyword.. — mx_code, Oct 08 '20 at 14:12
Does this answer your question? [Can I reference other properties during object declaration in JavaScript?](https://stackoverflow.com/questions/4618541/can-i-reference-other-properties-during-object-declaration-in-javascript) — John, Oct 08 '20 at 14:12
I want to achieve that with just a raw object.. (i.e. without any contructor/ prototype.) — mx_code, Oct 08 '20 at 14:13
There is no mechanism that allows an "under construction" object to be referenced inside the object initializer. You can partially construct the object and then use separate statements to add properties based on the values of other properties, or you can use the getter approach as suggested in the answer below to defer computation to the point where a property value is accessed. — Pointy, Oct 08 '20 at 14:19

Nina Scholz · Answer 1 · 2020-10-08T14:22:23.427

5

You could write y with a getter method:

The get syntax binds an object property to a function that will be called when that property is looked up.

console.log({
    x: 2,
    get y() { return this.x * 2 },
});

edited Oct 08 '20 at 14:22

answered Oct 08 '20 at 14:13

Nina Scholz

376,160
25
347
392

is this javascript? – mx_code Oct 08 '20 at 14:15
I mean if I try it from my side .. I'm only getting {x:2} .... nothing else! – mx_code Oct 08 '20 at 14:15
Assign it to an object like: `var obj = { x: 2, get y() { return this.x * 2 }};` Then call it: `obj[y]` – jmargolisvt Oct 08 '20 at 14:16
Actually I'm inside another class... so I thing this refers to that parent object... How can I solve this? – mx_code Oct 08 '20 at 14:17
How can I do it without assigning ?? – mx_code Oct 08 '20 at 14:17
What I want to achieve is that something like this```{ "x": 2, "y": 4 }```.... so what ever the value of x double that to get the value of y && the object should contain both x and y – mx_code Oct 08 '20 at 14:18
1

JavaScript provides no direct way of doing that. – Pointy Oct 08 '20 at 14:19
2

This is the best solution your going to get for what you want... For the context issue, you could create a function that returns the object as described here. – silencedogood Oct 08 '20 at 14:19
There must be a way... we can do this in a class and a function... – mx_code Oct 08 '20 at 14:22
There is no natural law of the universe that would insist that "there must be a way". In JavaScript, there is not a way. – Pointy Oct 08 '20 at 14:25
My bad... maybe I'm wrong! sorry... it may not be possible... I thought it would be! – mx_code Oct 08 '20 at 14:27
Note that the API when using a getter looks exactly like a property access... which appears to be what you want... `const o = { x: 2, get y() { return x * 2} }; console.log(o.y) // 4` – Ben Aston Oct 08 '20 at 15:19

score 0 · Answer 2 · answered Oct 08 '20 at 14:22

0

You could set your initial property first, and then the property that references it later:

const obj = {'x':2};
obj.y = obj.x*2;
console.log(obj);

answered Oct 08 '20 at 14:22

TKoL

13,158
3
39
73

1

Without any context about *why* you need to do it this way, it's hard to give you a good answer. – TKoL Oct 08 '20 at 14:23
I wanna do it in one go... I don't wanna assign to a variable.. – mx_code Oct 08 '20 at 14:26
Inside the object initializer, the object does not yet exist. The language provides no way of referring to the object until *after* the initializer is evaluated. – Pointy Oct 08 '20 at 14:27
My bad... maybe I'm wrong! sorry... it may not be possible... I thought it would be! – mx_code Oct 08 '20 at 14:27
If you wanna do it in one go, and you already know what the value of `x` is going to be as you're initializing it, then make `y` the value of 2 times that, without referencing x. – TKoL Oct 08 '20 at 14:33

score 0 · Answer 3 · answered Oct 10 '20 at 07:06

Just create a function that will take the first object as first arg and a callback in the second, then in the callback you would work on the things you set, return what you want, and merge it back to the original object.

console.log(mergeFunction({
      x: 2
   }, (o) => { return { 
      y: o.x * 2 
   }}
}));

Not pretty, but likely the best you can do if you want it in "one go".

You get to implement mergeFunction yourself.

How to reference an object inside itself?

3 Answers3