Js-Check "if user logged"

[hhcj]

Hi,
I'm looking display to customers the discount vouchers that are available at checkout.
I have a onepagecheckout module (it is not the default one with prestashop).
I have created a function that shows the discount vouchers through .tpl when the client is logged in.
My problem is that in the onepagecheckout is the form to login. Then when the client logs through this form, they already log in, but I do not know how to show the information at that moment. Because if I reload the page if it shows the information. But I need you to show it when you login.

Can someone tell me how to control if a client is logged into javascript and call my new function to show the information?

Sorry for my english.  I hope to have explained