Understanding the JavaScript Event Loop and Microtasks: A Beginner’s Guide

Introduction

JavaScript is known for being a single-threaded language, which means it can only do one thing at a time. But if that’s the case, how does it handle things like fetching data, responding to user interactions, or running animations without freezing the entire page? The answer lies in the Event Loop - a clever mechanism that makes JavaScript feel fast and responsive.

If you’ve ever wondered why setTimeout() doesn’t always run immediately, or why Promises resolve before setTimeout(), you’re about to get all the answers! In this guide, we’ll break down the JavaScript Event Loop and Microtasks in the simplest way possible.

What is the JavaScript Event Loop?

The Event Loop is like a manager that ensures JavaScript runs efficiently by deciding what to execute next. Since JavaScript can’t perform multiple operations at the same time, it needs a structured way to handle tasks.

Imagine a queue at a coffee shop:

1. You place an order (a task gets added to the queue).

2. The barista prepares your coffee (the JavaScript engine processes your task).

3. When your coffee is ready, your name is called (your task is completed).

Similarly, JavaScript organizes tasks and processes them in an order that ensures smooth performance.

How JavaScript Handles Tasks

In JavaScript, tasks are divided into synchronous and asynchronous operations:

1. Synchronous Tasks (Main Thread)

These are regular JavaScript statements that execute one after another, blocking the execution of further code until they finish.

console.log("Task 1");
console.log("Task 2");
console.log("Task 3");

Output:

Task 1
Task 2
Task 3

There’s nothing special here. JavaScript reads and executes these lines from top to bottom.

2. Asynchronous Tasks (Handled via Event Loop)

These include operations that take time to complete, such as fetching data, waiting for a timer, or responding to user actions.

console.log("Start");

setTimeout(() => console.log("Async Task"), 1000);

console.log("End");

Output:

Start
End
Async Task  (after 1 second)

Even though setTimeout was called first, JavaScript doesn’t wait for it - it moves on to the next statement while the timer runs in the background.

Microtasks vs. Macrotasks: Understanding the Priority System

Not all asynchronous tasks are treated the same. JavaScript has two types of task queues:

1. Microtasks (Higher Priority)

Microtasks include:

• Promises (.then, .catch, .finally)

• queueMicrotask()

• MutationObserver

They are executed right after the main script finishes and before any macrotasks.

2. Macrotasks (Lower Priority)

Macrotasks include:

• setTimeout, setInterval

• setImmediate (Node.js only)

• requestAnimationFrame

These are scheduled after microtasks are completed.

Execution Order Example

Let’s test this in code:

console.log("Start");
setTimeout(() => console.log("Macrotask - setTimeout"), 0);
Promise.resolve().then(() => console.log("Microtask - Promise"));
queueMicrotask(() => console.log("Microtask - queueMicrotask"));
console.log("End");

Output:

Start
End
Microtask - Promise
Microtask - queueMicrotask
Macrotask - setTimeout

👉 Even though setTimeout has 0 delay, it still runs after microtasks!

The Event Loop in Action (Step-by-Step Breakdown)

Let’s understand how the Event Loop works by breaking it into simple steps:

1. JavaScript runs the main script (top-to-bottom execution).

2. Microtasks are executed next (Promises, queueMicrotask).

3. Macrotasks run last (setTimeout, setInterval, etc.).

4. The Event Loop repeats the process, constantly checking for new tasks.

Here's a visual representation:

  ┌───────────────────────────────────────────┐
  │ JavaScript Code Execution (Main Thread)   │
  └───────────────────────────────────────────┘
             │
             ▼
  ┌───────────────────────────────────────────┐
  │ **Microtask Queue** (Higher Priority)     │
  │ Promises, queueMicrotask                  │
  └───────────────────────────────────────────┘
             │
             ▼
  ┌───────────────────────────────────────────┐
  │ **Macrotask Queue** (Lower Priority)      │
  │ setTimeout, setInterval, setImmediate     │
  └───────────────────────────────────────────┘

Common Misconceptions about the Event Loop

1. "setTimeout(0) runs immediately" ❌

   • No, it waits until the main script and all microtasks are done first.

2. "JavaScript runs everything at the same time" ❌

   • No, it runs one task at a time, handling async tasks using the event loop.

3. "Microtasks and macrotasks are the same" ❌

   • No, microtasks run first, making Promises faster than setTimeout.

Conclusion

The JavaScript Event Loop is what makes JavaScript non-blocking and efficient. It ensures that tasks are handled in the right order - synchronous tasks first, microtasks next, and macrotasks last. This allows JavaScript to handle user interactions, API requests, and timers without freezing the page.

Understanding this concept will help you debug performance issues, optimize asynchronous operations, and write better JavaScript code. So next time you see a setTimeout, Promise, or async/await, you'll know exactly how JavaScript is handling them! 🚀