function *range(max, step) {
  var count = 0;
  step = step || 1;

  for (var i = 0; i < max; i += step) {
    count++;
    yield i;
  }

  return count;
}

var gen = range(20, 3);
var values = [];

for (var value of gen) {
  values.push(value);
}

assert.deepEqual(values, [0, 3, 6, 9, 12, 15, 18]);