CODEWITHSUNDEEP
javascript
Vineet 'DEVIN' Dev
Vineet 'DEVIN' Dev

Reputation: 1223

Copy by reference

As per my knowledge, objects in JavaScript are copied by reference and not value. But the below code that I wrote is not working as expected. Maybe I don't know the proper behavior. Please provide insight and possible solution to avoid cases like this.

My code:

var a = {
    obj:{u:1,v:2,w:3},
    setobj:function(objj){
        this.obj = objj
    }
};

var b = a;
var m = b.obj
console.log(m);

It prints - {u:1,v:2,w:3},

then-

var c = a;
c.setobj({x:4,y:5});
console.log(c.obj);  //prints {x:4,y:5}
console.log(a.obj); //prints {x:4,y:5}
console.log(b.obj); //prints {x:4,y:5}

but:

console.log(m); //prints {u:1,v:2,w:3}

I was expecting m to contain the reference of b.obj, where b contains reference of a. The latter is working but the former(m's reference) is not working. Please provide an insight over this and tell me what am I missing. Is it very silly assumption of mine or is there something that I don't know?

Upvotes: 1

Views: 86

Answers (2)

Mazaher Bazari
Mazaher Bazari

Reputation: 421

{u:1,v:2,w:3} is a standalone object and has its reference address. when you assign obj:{u:1,v:2,w:3} , obj reference address is equal to {u:1,v:2,w:3} address. when you do c.setobj({x:4,y:5}); a new address will be assigned to obj.

your code is equal to:

var anotherObject= {u:1,v:2,w:3}
var a = {
obj:anotherObject,
setobj:function(objj){
    this.obj = objj
    }
};

Upvotes: 2

GolezTrol
GolezTrol

Reputation: 116190

The object is not copied, but the reference to the object is copied. The references themselves are values.

So, when you write m = b.obj, m points directly to the object 1,2,3. m is not a reference to b.obj, but a reference to (incidentally) the same object as to which b.obj is a reference.
So m and b.obj are both (now unrelated) references to the same object 1,2,3.

Then, when you overwrite b.obj, that reference changes to the object 4,5, but m is still a reference to the original object.

Upvotes: 6

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