Streaming data with async generators

async-streaming.ts

async function returnWait<T>(timeToDelay: number, returnValue: T): Promise<T> {
  return new Promise((resolve) => setTimeout(() => resolve(returnValue), timeToDelay));
}

const encoder = new TextEncoder();

async function* iterator() {
  let index = 0;
  if (index === 0) {
    const thing = await returnWait(1000, { hello: "first" });
    yield encoder.encode("[" + JSON.stringify(thing));
    ++index;
  }

  while (index !== 1000) {
    const thing = await returnWait(1000, { hello: "middle" });
    index++;
    if (index === 1000) {
      yield encoder.encode(JSON.stringify(thing) + "]");
    } else {
      yield encoder.encode(JSON.stringify(thing));
    }
  }
}
const generator = iterator();

const streamResponse = new ReadableStream({
  async pull(controller) {
    const { value, done } = await generator.next();

    if (done) {
      controller.close();
    } else {
      controller.enqueue(value);
    }
  },
});

return new Response(streamResponse, {
  status: 200,
  headers: {
    "Content-Type": "application/json",
  },
});