What does shorthand “index >= 0 && count++” do?

Question

I was killing time reading the underscore.string functions, when I found this weird shorthand:

function count (str, substr) {
  var count = 0, index;
  for (var          


        

Answer 1

It's equivalent to:

if (index >= 0) {
    count = count + 1;
}

&& is the logical AND operator. If index >= 0 is true, then the right part is also evaluated, which increases count by one.
If index >= 0 is false, the right part is not evaluated, so count is not changed.

Also, the && is slightly faster than the if method, as seen in this JSPerf.