var chars = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXTZabcdefghiklmnopqrstuvwxyz";
var string_length = 7;
var randomstring = '';
for (var i=0; i<string_length; i++) {
	var rnum = Math.floor(Math.random() * chars.length);
	randomstring += chars.substring(rnum,rnum+1);
}