Hi, I was reading HERE that since WP 2.8 a global variable "ajaxurl" is defined to be used in ajax calls.
The fact is, before starting writing the code I tested the value of it but it says it's not defined. My app already uses JQuery.
In the console I get:
ajaxurl is not defined alert( ajaxurl );
edit: Ok reading more carefully the link, it says that on the frontend that variable is not automatically defined.