[javascript] Sample of runner queue to ensure which async function will run non parallel just after another

slow-running-queues.js

// Helper: wait for a given number of milliseconds
function wait(ms) {
  return new Promise(resolve => setTimeout(resolve, ms));
}

// Simple FIFO queue with O(1) enqueue/dequeue (amortized)
class Queue {
  constructor() {
    this.items = [];
    this.head = 0;
  }

  enqueue(item) {
    this.items.push(item);
  }

  dequeue() {
    if (this.head >= this.items.length) return null;
    const item = this.items[this.head++];
    // optional: cleanup memory when too many items were dequeued
    if (this.head > 50 && this.head * 2 > this.items.length) {
      this.items = this.items.slice(this.head);
      this.head = 0;
    }
    return item;
  }

  get length() {
    return this.items.length - this.head;
  }
}

// Create a serialized version of an async function.
// Ensures only one call runs at a time, with a delay between each call.
function serialize(fn, intervalMs = 3000) {
  const queue = new Queue();
  let isProcessing = false;

  async function processQueue() {
    if (isProcessing) return;
    isProcessing = true;

    while (queue.length > 0) {
      const { args, resolve, reject } = queue.dequeue();
      try {
        const result = await fn(...args);
        resolve(result);
      } catch (error) {
        reject(error);
      }
      await wait(intervalMs);
    }

    isProcessing = false;
  }

  // Returned function automatically enqueues calls
  return function (...args) {
    return new Promise((resolve, reject) => {
      queue.enqueue({ args, resolve, reject });
      processQueue();
    });
  };
}

async function slowFunction(data) {
  console.log(data, 'started', 'at', new Date().toISOString());
  await wait(1000);
  console.log(data, 'finished', 'at', new Date().toISOString());
  return data;
}

// Example usage:
const queuedFunction = serialize(slowFunction, 3000);

queuedFunction("a");
queuedFunction("b");
queuedFunction("c");
queuedFunction("d");
queuedFunction("e");
queuedFunction("f");
queuedFunction("g");
queuedFunction("h");
queuedFunction("i");

// a finished at 2025-10-31T19:07:25.249Z
// b started at 2025-10-31T19:07:28.250Z
// b finished at 2025-10-31T19:07:29.251Z
// c started at 2025-10-31T19:07:32.251Z
// c finished at 2025-10-31T19:07:33.252Z
// d started at 2025-10-31T19:07:36.252Z
// d finished at 2025-10-31T19:07:37.253Z
// e started at 2025-10-31T19:07:40.253Z
// e finished at 2025-10-31T19:07:41.253Z
// f started at 2025-10-31T19:07:44.254Z
// f finished at 2025-10-31T19:07:45.254Z
// g started at 2025-10-31T19:07:48.255Z
// g finished at 2025-10-31T19:07:49.256Z
// h started at 2025-10-31T19:07:52.256Z
// h finished at 2025-10-31T19:07:53.257Z
// i started at 2025-10-31T19:07:56.408Z
// i finished at 2025-10-31T19:07:57.408Z