fib.js

function* fib() {
  var i = 0,
      j = 1,
      res;
  while (true) {
    res = i + j;
    i = j;
    j = res;
    yield res;
  }
}

var g = fib();

console.log(g.next().value);
console.log(g.next().value);
console.log(g.next().value);
console.log(g.next().value);
console.log(g.next().value);