JavaScript: Store Object as Fixed Value?

Question

I've noticed this behavior when writing my JavaScript, and I haven't been able to figure out why:

Below is some code to reproduce the behavior in question.

var o1 = {
  num: 1
}
var o2 = o1;
o2.num = 2;
alert(o1.num);

Expected Result: The browser alerts 1, because I only changed a property of the o2 object, not the o1 object.

Actual Result: The browser alerts 2, because it seems o1 is equal to o2.

I'm not really sure what's going on. How can I fix the code so it alerts 1 rather that 2 (assuming that o1 hasn't changed)?

Much thanks in advance.

Answer 1

Because both variables reference the same object. Objects are not cloned/copied on variable assignment. You would have to do this yourself.

JavaScript behaves the same way like any (most) other OO languages in this case.