Object and array with self referencing

Question

I am very interested in understanding JavaScript behavior.

Object :

object = {object : window.object};
object = {object : window.object};

console.log(object === object); // true
console.log(object.object === object); // false
console.log(object.object === object.object); // true
console.log(object.object.object === object.object); // false

Array :

array = [window.array];
array = [window.array];

console.log(array === array); // true
console.log(array[0] === array); // false
console.log(array[0] === array[0]) // true
console.log(array[0][0] === array[0]) // false

Why

object.object.object === object.object

returns false ???

Why define the variable twice, overwriting it? Edit: ok ;) It does not work without them — blex
– blex, Commented Jul 2, 2015 at 20:10

Barmar · Accepted Answer · 2015-07-02 20:22:45Z

2

After the first assignment, you have the following:

object = { object: undefined }

The second assignment creates a new object, whose object property contains the previous value of window.object, and assigns this to object. So now you have:

object = { object: { object: undefined } }

So at this time, object.object is { object: undefined }, which is not the same as object.

Similar things happen with the array example.

If you want to create a self-referential object, you need to do:

object = {};
object.object = object;

This doesn't create a new object in the second assignment, it modifies the original object. Then you could do:

console.log(object.object.object.object === object); // true

For an array, it would be:

array = [];
array[0] = array;
console.log(array[0][0][0][0][0] === array); // true

edited Jul 2, 2015 at 20:22

answered Jul 2, 2015 at 20:16

Barmar

789k57 gold badges555 silver badges669 bronze badges

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

Comments

Pointy · Accepted Answer · 2015-07-02 20:14:32Z

1

There are two assignments to window.object. The first one creates a new object and assigns it to window.object. That one has a property called "object" whose value is undefined, because at the time the object literal is evaluated window.object is undefined.

The second assignment instantiates a new object. That one also has a property called "object", whose value is the object created in the first assignment.

Thus, window.object is not the same object as window.object.object.

The key is that in an assignment like this:

object = { object: window.object };

The value of the "object" property in the object literal is evaluated before the assignment takes place.

edited Jul 2, 2015 at 20:14

answered Jul 2, 2015 at 20:13

Pointy

415k62 gold badges600 silver badges631 bronze badges

2 Comments

user2594803 Over a year ago

So there are two objects ?

Pointy Over a year ago

Yes, there are two objects. Why wouldn't there be? Every distinct object literal expression creates a new object.

Anoop · Accepted Answer · 2015-07-02 20:23:06Z

Let explain it line by line

    object = {object : window.object};
    //here object.object is undefined and object new object so object !== object.object
    //lets obj1 = {object: undefined}

    object = {object : window.object};
    //here object is new instance of Object while object.object is equal to older object
   //basically object != object.object 
    // obj2 = {object: obj1}
    console.log(object === object); // true because same object obj2 == obj2
    console.log(object.object === object); // false because two different object  as obj1 != obj2
    console.log(object.object === object.object); // true because same object obj1 == obj1
    console.log(object.object.object === object.object); // false because undefined != obj1

Collectives™ on Stack Overflow

Object and array with self referencing

3 Answers 3

Comments

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related