async_iterator.ts

const CHUNK_SIZE = 50;

class FetchUsers implements AsyncIterable<string> {
  #users: string[]; 
  #chunk: number;

  constructor() {
    this.#chunk = 0;
    this.#users = [];
  }

  #generateChunk() {
    if (this.#chunk >= 5) {
      this.#users = [];
      return;
    }
    this.#users = new Array(CHUNK_SIZE);
    for (let i = 0; i < CHUNK_SIZE; i++) {
      const user = `User ${i + (this.#chunk * CHUNK_SIZE)}`;
      this.#users[i] = user;
    }
    this.#chunk += 1;
  }

  [Symbol.asyncIterator]() {
    const self = this;
    return {
      async next(value?: any): Promise<IteratorResult<string>> {
        if (!self.#users.length) {
          self.#generateChunk();
        }
        const user = self.#users.shift();
        if (user === undefined) {
          return { done: true, value: null };
        }
        return { done: false, value: user };
      }
    }
  }
}

async function main() {
  const users = new FetchUsers();

  for await (const user of users) {
    console.log(`Got user: ${user}`);
  }
}

main().catch(console.error);