How object referencing works in JavaScript?

Question

I'm new to JavaScript and I thought objects are passed by reference.

The output I expected was:

{ one: 1 } { one: 1 }
{ two: 2 } { two: 2 }

Output obtained:

{ one: 1 } { one: 1 }
{ two: 2 } { one: 1 }

When b is referencing address of a, why is b still { one: 1 }

var a = {one:1}
var b = a

console.log(a,b)

a = {two:2}

console.log(a,b)

This really doesn't have to do with pass-by-reference either, since there is no passing involved in this question, but the duplicate explains this behavior anyway. — Heretic Monkey, Sep 07 '19 at 18:09
In js you cannot have object value. You always have object referens. So you actually passing references by value. — j2ko, Sep 07 '19 at 18:19

score 5 · Answer 1 · answered Sep 07 '19 at 18:05

when you did b = a, so now the b holds the same reference as of a

But when you assign a new value to a

a = { one : 2 }

It created a new memory reference for the value ( { one : 2 } ) and tag it with a variable, so you can refer to the memory reference using a and get the value. but b still holds the initial reference of a which has value {one : 1}

score 1 · Answer 2 · answered Sep 07 '19 at 18:10

Let's analyze this one by one:

var a = { one: 1 }  // 'a' points now to (example) location: #fff
var b = a        // 'b' points to the same location: #fff

Now, when you are asigning a new object to a:

a = { two: 2 }

a now points to new location in memory (say, #ff1) and it doesn't affect b's location, which still points to #fff. That is why the last console.log(a,b) call gives you { two: 2 } { one: 1 } output.

score 1 · Answer 3 · answered Sep 07 '19 at 18:11

When b is referencing address of a, why is b still { one: 1 }

Because b is not referencing the address of a, but both of them are referencing the address of object { one: 1 }. Then a starts referencing the address of a brand new, different object, { two: 2 }, and absolutely nothing changes about b, or the first object, { one: 1 }, which still remains intact and existing, exactly because b needs it.

How object referencing works in JavaScript?

3 Answers3

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