How I Mastered JavaScript Event Loop and Concurrent Model | HackerNoon

For some time, JavaScript events loop used to confuse me.

JavaScript is single-threaded, but still it handles thousands of things at the same time (e.g user clicks, network calls and even setting timers). I have always wondered how that worked. After studying and implementing it for a while, it has improved how I build web applications.

How I think of it

Imagine a chef (the primary thread) in the kitchen, instead of cooking one particular dish(order) completely, he

1. Takes an order (Function)
2. Starts cooking (Start async operation)
3. Starts another order while waiting for the food to get simmered
4. Returns to check the simmering order when possible

This is how JavaScript handles concurrency despite being single-threaded.

How I applied this to my projects

• API calls without Freezing the UI

  // Previously: Freezing UI while waiting for data 
  function loadUserData() {
     const data = fetchDataSync('/api/user'); // UI freezes here
     updateUI(data);
     }

  // Now: Non-blocking with callbacks 
  async function loadUserData() {
     const response = await fetch('/api/user');
     const data = await response.json();
     updateUI(data);
     }

• Doing the hard task only once after things stop changing

  When implementing a live search feature

  function waitBeforeSearch() {
    let timer;
   
    return function (text) {
      clearTimeout(timer);
      timer = setTimeout(() => performSearch(text), 300);
    };
  }

Important parts to note

• Call Stack: Where the function executes (the “chef cooking”)
• Callback Queue: Tasks waiting (Orders)
• Microtask Queue: Higher Priorities (Promises, Observers)

Here is a rule to remember

Never block the call stack with synchronous long-running operations. If you need to process long tasks, you can

• Use web workers
• Break tasks into smaller chunks with setTimeOut or requestIdleCallBack
• User async/await for Input-Output operations

Getting to understand the JavaScript event loop and concurrency model has helped me to build more responsive applications. No freezing interfaces even during heavy processing because I now properly schedule task in different queues. Mastering the event loop and concurrency model means understanding exactly when and why your code executes, which make a better and more responsive application.