Programming :: Trying To Understand This Function In JavaScript

In my book, this is an example of a recursive algorithm:

var reverseArray = function(x,indx,str) {
return indx == 0 ? str : reverseArray(x,--indx,(str+= " " + x[indx])) ;; }
var arr = new Array('apple','orange','peach','lime');
var str = reverseArray(arr,arr.length,"");
alert(str) ;

I'm trying to understand it, I'm stuck at the (str+= " " + x[indx])) part - that part of the function definition expects an ARGUMENT, am I right? What's being passed to it is a bit of PROGRAM, the str+= stuff. So - how is it working?