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 the sequence at 1000
      if (n > 1000)
        break;
      print(n);
    }

I love how the only comment is explaining what "if (n > 1000) break;" does