Using comma in Javascript variable declaration

Question

I just came across the following code:

function showMatch(str, reg) {
var res = [], matches
 while(true) {
  matches = reg.exec(str)
  if (matches === null) break
   res.push(matches[0])
  if (!reg.global) break
alert(res)
}

Can anybody please explain the second row? Does

var res = [], matches 

equal

var res=[]; res=matches

or

var res=[]; var matches=[]

?

I guess the second answer is correct? I find this little confusing...

Answer 1

It equivalent to

var res = []; 
var matches; 

where matches is undefined