javascript equal to operator

Question

What is the reason for the following code resulting to false?

{} === {}
//false

what is the reason for this code turning true?

 var obj = {};
 obj === obj
  //true

plz answer in a simple and concise language

1) Different objects. (2) Same object. Simple and concise enough? — James Allardice, Apr 02 '13 at 15:20
http://stackoverflow.com/questions/359494/javascript-vs-does-it-matter-which-equal-operator-i-use?rq=1 — Josh Lee, Apr 02 '13 at 15:21

score 4 · Answer 1 · answered Apr 02 '13 at 15:20

Comparisons of objects don't check if the objects have the same content, they check if they are the same object. Each {} creates a new object. Your first test tests to see if a new object is the same object as a different new object. Your second test compares the created object to itself.

From the the documentation for comparison operators:

An expression comparing Objects is only true if the operands reference the same Object

Or, from the language specification:

Step 7: Return true if x and y refer to the same object. Otherwise, return false.

score 1 · Accepted Answer · answered Apr 02 '13 at 15:20

1

In the first case, you are creating two objects and comparing them. In the second case you are comparing the same object.

answered Apr 02 '13 at 15:20

Julien Grenier

3,364
2
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that means if two different string are compared like above will it out put false?for e.g "sa"==="sa" – Maizere Pathak.Nepal Apr 02 '13 at 15:30
1

@Maizere No because JS won't recreate new String object each time when you use the string literal.But note that new String("a") === new String("a") will return false. – Julien Grenier Apr 02 '13 at 15:38

score 0 · Answer 3 · answered Apr 02 '13 at 15:21

When you do {} you're creating an (empty) object literal (a new object with no properties). When you're doing

{} === {}

you're comparing two entirely different objects, so it returns false.

However, when you do:

var obj = {};
obj === obj

you're comparing the same object, so it returns true.

score 0 · Answer 4 · answered Apr 02 '13 at 15:21

0

In second you are comparing an object with itself that is very logic to be equal.

At first you are not comparing the same things, you try to compare things that show the same.

answered Apr 02 '13 at 15:21

Mhche

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14

score 0 · Answer 5 · answered Apr 02 '13 at 15:21

{} creates a new object.

=== checks that they are actually the same object. Although they are both new empty objects, they are actually different empty objects.

To see why this matters, try this:

var a = {};
var b = {};
var c = a;
a.key = 'value';
console.log(b);
console.log(c);

You will see that because a and b are different empty objects, changing a doesn't change b. Because c is actually the same empty object as a, putting something into a also puts it into c.

javascript equal to operator

5 Answers5