Prefix Image with 0001a, 0002b and so on

I am trying to fix a problem. The code in the file is

var randomstring = (Math.random() + 1).toString(36).substring(7) + n;


The above code generate suffix as -adfs234sdfsdf and -asf3254sadf23 means random characters + numbers combination
Example: 
abc-wallpaper-adfs234sdfsdf.jpg
def-wallpaper-asf3254sadf23.jpg

What I am trying to get is prefix as 0001a-abc-wallpaper.jpg instead of -asf3254sadf23

So, what coding should I use to get 0001a, 0002b as the result using var randomstring = (Math.random() + 1).toString(36).substring(7) + n; function ?