What are Async Generators and Async Iterators?

Async generators combine generator syntax with async/await — they yield values asynchronously and are consumed with for await...of.

// Async generator — paginated API fetcher
async function* fetchPages(url) {
  let page = 1;
  while (true) {
    const res = await fetch(`${url}?page=${page++}`);
    const { items, hasMore } = await res.json();
    yield items; // pause, give back items, resume on next iteration
    if (!hasMore) return; // done
  }
}

// Consume with for await...of
async function loadAll() {
  const allItems = [];
  for await (const items of fetchPages('/api/data')) {
    allItems.push(...items);
    if (allItems.length >= 100) break; // can stop early!
  }
  return allItems;
}

// Streaming data (Fetch Streams API)
async function* streamLines(url) {
  const reader = (await fetch(url)).body.getReader();
  const decoder = new TextDecoder();
  let buffer = '';
  while (true) {
    const { done, value } = await reader.read();
    if (done) { if (buffer) yield buffer; return; }
    buffer += decoder.decode(value);
    const lines = buffer.split('\n');
    buffer = lines.pop();
    for (const line of lines) yield line;
  }
}