Javascript – How does variable assignment work in JavaScript


So I was playing around the other day just to see exactly how mass assignment works in JavaScript.

First I tried this example in the console:

a = b = {}; = 'bar';

The result was "bar" being displayed in an alert. That is fair enough, a and b are really just aliases to the same object. Then I thought, how could I make this example simpler.

a = b = 'foo';
a = 'bar';

That is pretty much the same thing, isn't it? Well this time, it returns foo not bar as I would expect from the behaviour of the first example.

Why does this happen?

N.B. This example could be simplified even more with the following code:

a = {};
b = a; = 'bar';

a = 'foo';
b = a;
a = 'bar';

(I suspect that JavaScript treats primitives such as strings and integers differently to hashes. Hashes return a pointer while "core" primitives return a copy of themselves)

Best Answer

In the first example, you are setting a property of an existing object. In the second example, you are assigning a brand new object.

a = b = {};

a and b are now pointers to the same object. So when you do: = 'bar';

It sets as well since a and b point to the same object.


If you do this instead:

a = 'bar';

you are saying that a points to a different object now. This has no effect on what a pointed to before.

In JavaScript, assigning a variable and assigning a property are 2 different operations. It's best to think of variables as pointers to objects, and when you assign directly to a variable, you are not modifying any objects, merely repointing your variable to a different object.

But assigning a property, like, will modify the object that a points to. This, of course, also modifies all other references that point to this object simply because they all point to the same object.