js2 auto complete not working on emacs

Question

I started using Emacs a couple weeks ago. I installed js2-mode on emacs and works like a charm, but now I would like to have autocompletion when working in there.

I already installed ac-js2-mode, but when I work in js2-mode I don't see any completion at all. Any ideas?

Edit: I installed and enabled auto-complete-mode, but when I'm typing a variable it says in the minibuffer the following error:

"auto-complete error: (void variable ac-source-js2)