Javascript pass by reference or value?

Question

I have seen this question being asked multiple times but I have a specific example to clarify.

var a = {animal: 'cat'}; var b = a; a.animal = 'bear';

Here, I see b.animal will give an output "bear". Why is that? And how do I retain the original reference "cat" for variable b?

You have copied a reference from a to b, but they both point at the same object. If you want to copy the object, then you need to copy each property of that object to a new object. — Matt Burland
– Matt Burland, Commented Mar 4, 2014 at 16:14
Take a look at this question: stackoverflow.com/q/122102/522479 — Cobra_Fast
– Cobra_Fast, Commented Mar 4, 2014 at 16:15
It's by-value, but the value is an object reference. So, b is a copy of a, but they both still refer to the same object. Is JavaScript a pass-by-reference or pass-by-value language? — Jonathan Lonowski
– Jonathan Lonowski, Commented Mar 4, 2014 at 16:16

Community · Accepted Answer · 2017-05-23 10:31:57Z

Yes, both refer to the same object.

May be you are confused because in Javascript, you usually don't use the "new" keyword on built-in types, even if you can.

You could also declare "a" like this :

var a = new Object(); a.animal = "cat";

Now it is maybe more clear why "b" change when "a" change.

If you "b" have its own property then you have to clone "a".

Community · Accepted Answer · 2017-05-23 10:26:11Z

You should clone the object instead of copying the reference (b = a);

here's a question where this problem is very well explained: How do I correctly clone a JavaScript object?

nullpotent · Accepted Answer · 2014-03-04 16:16:29Z

'Primitives' are copied, objects are not (that is, the reference to it is copied). If you need to protect the property, define it writeable:false via defineProperty

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