I'm coding a chrome extension. My extension is for free but i want people to know better about the product that I provide. So, when a user used my extension 5 times or more, I want to get the user's mail adress (i propose him to enter it). Here I come to the problem : before asking the user his mail adress, I have to count the number of times he clicked. popup.js open itself each time a user open an extension. Here my algorithm :

-> Each time popup.js open :

id = ? from the user

if id not in dico :

add ? key to the dico, with value [1, false]

else

add 1 to dico.id[0]

if dico.id[0] >= 5 and dico.id[1] == false

    ask the mail

        if the user entered his mail

            dico.id[1] = true

            add the mail to my database

My question : what could be "?" ? I want something stable, like the IP (not stable because of the VPN), or the MAC adress (impossible acces). I also want to be able to get "?" with javascript.

Thanks.