Question

What does the following javascript do?

var groups = countrylist.split(',');    
for( var i = -1, group;  group = groupsCounty[++i]; ){
  ...
}

Answer 1

With i starting at -1
increase i by 1
get the ith element from the groupsCounty array
if there is no such element: stop
otherwise: assign it to group and carry on (with whatever "…" is)

It’s an optimised version of:

for (var i = 0; i < groupsCounty.length; i++; ){
    var group = groupsCounty[i];
    …
}