Design Pattern - Async Iterator

design-pattern-async-iterator.js

class MyAsyncIterator {
  constructor() {
    this.queue = []; // To hold actual values
    this.waitlist = []; // To hold the callbacks
  }

  /**
   * If the waitlist contains an item, remove it and execute it.
   * Otherwise, add the value to the queue.
   */
  push(val) {
    if (this.waitlist.length > 0) {
      const firstCallback = this.waitlist.shift();
      firstCallback(val);
    } else {
      this.queue.push(val);
    }
  }
  /**
   * If the queue has an item, remove it and execute it with RESOLVE.
   * Otherwise, add the callback to the waitlist.
   */
  next() {
    // Since it's gonna need a promise, we will return a Promise.
    return new Promise((resolve) => {
      // Something is in queue, remove that value
      if (this.queue.length > 0) {
        const firstValue = this.queue.shift();
        resolve(firstValue);
      } else {
        // Queue is empty, hence add callback to waitlist
        this.waitlist.push(resolve);
      }
    });
  }
}

function start() {
  const iterator = new MyAsyncIterator();

  iterator.push(1);
  iterator.push(2);

  iterator.next().then((val) => {
    console.log(val);
  }); // 1
  iterator.next().then((val) => {
    console.log(val);
  }); // 2
  iterator.next().then((val) => {
    console.log(val);
  }); // console 3 after 2000 milliseconds.

  setTimeout(() => {
    iterator.push(3);
  }, 2000);
}

start();