Design Pattern - Async Queue

concurrency-pattern-async-queue.js

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

  /**
   * If the callbacks array has something, remove it and execute it.
   * Otherwise, add the value to the values array.
   */

  push(val) {
    if (this.callbacks.length > 0) {
      let firstCallback = this.callbacks.shift();
      return firstCallback(val);
    } else {
      this.values.push(val);
    }
  }

  /**
   * If the values array has something, remove it and execute it with RESOLVE.
   * Otherwise, add the callback to the callbacks array.
   */

  next() {
    // Since it's gonna need a promise, we will return a Promise.
    return new Promise((resolve) => {
      // Something is in values, remove that value
      if (this.values.length > 0) {
        let firstValue = this.values.shift();
        return resolve(firstValue);
      } else {
        // Values array is empty, hence add callback to callbacks array
        this.callbacks.push(resolve);
      }
    });
  }
}

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

  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();