❗️Что будет на выходе:

function* fibonacci() {
  let [a, b] = [0, 1];
  while (true) {
    yield a;
    [a, b] = [b, a + b];
  }
}

function* takeWhile(generator, predicate) {
  for (const value of generator) {
    if (!predicate(value)) break;
    yield value;
  }
}

const fib = fibonacci();
const smallFibs = takeWhile(fib, x => x < 20);
console.log([...smallFibs].join(','));

Ответ: 0,1,1,2,3,4,8,13

JavaScript test | #JavaScript