obj == JSON.parse(JSON.stringify(obj)) is false

Question

I was looking at the JavaScript console in Chrome and noticed something strange, even though they appear the same, obj and JSON.parse(JSON.stringify(obj)) are not the same. Why is that?

var obj = {test:'this is a test', another: {omg:'ay dios mio', check:true}};
console.log(obj, JSON.parse(JSON.stringify(obj)));
console.log(obj == JSON.parse(JSON.stringify(obj)));

They look identical but return false when you check equality. Why is that?

Not *really* a duplicate, but the answer there answers your question too. — Lix, Jul 20 '14 at 11:37
If you convert both to strings `JSON.stringify()` then you'll see their values are equal. — Lix, Jul 20 '14 at 11:38
[MDN: Equality operators](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Comparison_Operators#Equality_operators) `[...]If both operands are objects, then JavaScript compares internal references which are equal when operands refer to the same object in memory.` — t.niese, Jul 20 '14 at 11:39

score 9 · Accepted Answer · answered Jul 20 '14 at 11:37

9

They're not equal for the same reason this returns false:

({omg:'ay dios mio', check:true}) == ({omg:'ay dios mio', check:true})

You're not comparing the values inside the object, but the object references. They'll be different.

answered Jul 20 '14 at 11:37

Overv

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score 5 · Answer 2 · answered Jul 20 '14 at 11:37

5

The objects are testing for REFERENCES.

While primitives are testing for VALUE.

answered Jul 20 '14 at 11:37

Dávid Szabó

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score 5 · Answer 3 · edited May 23 '17 at 11:53

Because the obj does not reference the parsed object in memory. So these are 2 different declarations. If you do this:

var a = [ 10 ],
    b = [ 10 ];

Then there are 2 instances of arrays with the same values, but that doesn't make them the same array. So a != b, even though 10 == 10. You can increase the value of a[0] to 15, but that doesn't change the value of b[0] to 15.

Therefore, if you want to compare objects, you have to loop through them and check if the values of the objects are the same.

A function to compare (borrowed from jQuery object equality )

$.fn.equals = function(compareTo) {
  if (!compareTo || this.length != compareTo.length) {
    return false;
  }
  for (var i = 0; i < this.length; ++i) {
    if (this[i] !== compareTo[i]) {
      return false;
    }
  }
  return true;
};

But the way they reference are the same, I could have made it objects with just 1 property. That's true. — Niels, Jul 20 '14 at 11:40
Yes (because arrays are objects). It's just, introducing further non-relevant factors tends to obscure the explanation... — T.J. Crowder, Jul 20 '14 at 11:41

obj == JSON.parse(JSON.stringify(obj)) is false

3 Answers3