generator.js

function* enumerate(iterable) {
  let i = 0;
  for (const item of iterable) {
    yield [i++, item];
  }
}

function* withIsLast(iterable) {
  const iterator = iterable[Symbol.iterator]();
  let curr = iterator.next();
  let next = iterator.next();
  while (!curr.done) {
    yield [next.done, curr.value];
    [curr, next] = [next, iterator.next()];
  }
}

for (const [isLast, [i, char]] of withIsLast(enumerate("hello"))) {
  console.log(isLast ? 'last:' : '', i, char);
}