How to write a function that might not return anything

foo.coffee

foo = (a, b, plusOne = true) ->
  out = a + b
  out += 1 if plusOne

foo.js

var foo;

foo = function(a, b, plusOne) {
  var out;
  if (plusOne == null) {
    plusOne = true;
  }
  out = a + b;
  if (plusOne) {
    return out += 1;
  }
};

lol, yep