ES6 generators function

d.js

function* double(x) {
  while(true) {
    x = x * 2;
    yield x;
 }
}

var g = double(2);
console.log(g.next()); // # => 4
console.log(g.next()); // # => 8
console.log(g.next()); // # => 16

g.js

function* count() {
  yield 1;
  yield 2;
  yield 3;
  yield 4;
  return 5;
}

var c = count();
console.log(c.next()); // # => 1
console.log(c.next()); // # => 2
console.log(c.next()); // # => 3
console.log(c.next()); // # => 4
console.log(c.next()); // # => 5