JS remove everything after the last occurrence of a character

Here you are:

var URL = "http://stackoverflow.com/questions/10767815/remove-everything-before-the-last-occurrence-of-a-character";
alert(URL.substring(0, URL.lastIndexOf("/") + 1));

Hope this helps.

Seems like a good case for a regular expression (can't believe no one has posted it yet):

URL.replace(/[^\/]+$/,'')

Removes all sequential non–forward slash characters to the end of the string (i.e. everything after the last /).