For..of + iterator

let fibonacci = {
  [Symbol.iterator]() {
    let pre = 0, cur = 1;
    return {
      next() {
        [pre, cur] = [cur, pre + cur];
return { done: false, value: cur }
      }
    }
  }
}
for (var n of fibonacci) {
  // truncate sequence at 1000
  if (n > 1000) break;
  console.log(n);
}

For iterating over generators and arrays. See: For..of iteration