[pcwalton]

You can generally do that in JS as well. (Added properties are sometimes called "expandos".)

[eva1984]

True. Just find out. But Chrome's console yield undefined to me...Weird.

Edit: find the reason. It is because I am trying to assign a property to int. Seems JS won't allow this to primitive type?

Edit: Python won't allow assignment to primitive type bject either. The handling is the same.