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 the same as:

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

JavaScript will evaluate the left side (index >= 0), if it's false the && (AND) will short circuit (since false AND anything is false), thus not running `count++.

If it's (index >= 0) true, it evaluates the right side (count++), then it just ignores the output.