The memoized version does not even remotely work.

  a = fib(5);
  for (i = 0; i < 7; ++i) console.log(a.next());

gives the expected

  Object {value: 0, done: false}
  Object {value: 1, done: false}
  Object {value: 1, done: false}
  Object {value: 2, done: false}
  Object {value: 3, done: false}
  Object {value: undefined, done: true}
  Object {value: undefined, done: true}

But then

  b = fib(3)
  for (i = 0; i < 5; ++i) console.log(b.next());

gives the completely wrong:

  Object {value: 1, done: false}
  Object {value: 0, done: false}
  Object {value: 1, done: false}
  Object {value: 1, done: false}
  Object {value: undefined, done: true}