Understanding JavaScript's Event Loop, Microtasks, and Macrotasks: A Beginner's Guide

JavaScript is a powerful language often used in web development, and one of its core concepts is how it handles asynchronous operations. To make this concept clearer for beginners, we'll explore the Event Loop, and the roles of microtasks and macrotasks, complete with simple examples to demonstrate how JavaScript manages various operations.

Simplified Overview of the Event Loop

The Event Loop is a mechanism that JavaScript uses to handle execution of code, events, and messages in an efficient manner. Here’s an easy way to visualize it:

  1. Stack: This is where your JavaScript code starts executing, from the top down, like stacking blocks.
  2. Heap: Think of this as the memory store for objects your code creates.
  3. Queue: A list where tasks that need to be executed are lined up.

The Event Loop continuously checks the stack to see if there's anything that needs to be run. If the stack is empty, it looks at the queue to see if there are any waiting tasks to execute. This cycle allows JavaScript to perform tasks like updates to the UI, handle user interactions, or fetch data in the background without blocking the main thread.

Here's a simple example to explain how the JavaScript event loop works, particularly focusing on handling asynchronous events with setTimeout:

console.log('Start'); setTimeout(function() { console.log('Timeout Callback'); }, 1000); console.log('End');

In this example:

  1. console.log('Start'); is executed first, printing "Start" to the console.
  2. setTimeout schedules a callback function to be executed after 1000 milliseconds (1 second). However, it does not block the subsequent code from executing.
  3. console.log('End'); is executed immediately after scheduling the timeout, printing "End" to the console.
  4. After the main thread is clear and the 1 second delay has passed, the Event Loop checks if there are any tasks in the queue. It finds the callback from setTimeout and executes it, printing "Timeout Callback" to the console.

This sequence effectively demonstrates how the JavaScript event loop handles tasks and how it uses the task queue to manage asynchronous events like timeouts. The setTimeout callback is an example of a macrotask that gets processed after the currently executing script and all other pending tasks are completed. This ensures that the synchronous code runs immediately without waiting for the asynchronous code (like I/O operations or timers), which keeps the application responsive.

Microtasks vs. Macrotasks

What are Macrotasks?

Macrotasks are chunky pieces of tasks that include things like:

  • setTimeout: Delays a piece of code from running until at least the time you specify has passed.
  • setInterval: Continuously runs a piece of code after every specified interval.
  • Events and I/O operations: Like clicking a button or loading data from an external source.

Each macrotask is completed before moving on to the next one.

What are Microtasks?

Microtasks are small jobs that need to be done without interruption. These include:

  • Promise resolutions: Actions you promise to perform when something else is done.
  • queueMicrotask function: Directly adds microtasks.

Microtasks are processed faster and more frequently than macrotasks. They get processed after each macrotask, and before the JavaScript engine takes up the next macrotask.

Real-World Code Examples

Example 1: Using setTimeout as a Macrotask

Imagine you want to display a message on a webpage after 2 seconds.

setTimeout(() => { console.log('This message shows after 2 seconds'); document.getElementById('message').textContent = 'Hello there!'; }, 2000);

Explanation: This code sets up a timer that waits 2 seconds before executing. It then changes the text of an element on the page. This is a macrotask because it's scheduled to run independently of the main program flow.

Example 2: Using Promises as Microtasks

Let’s say you want to log a message right after a quick task, like updating a status.

Promise.resolve().then(() => { console.log('Promise resolved!'); document.getElementById('status').textContent = 'Update complete'; });

Explanation: This code creates a promise that resolves immediately, then immediately runs the code inside the .then() once the stack is clear. This is a microtask because it’s a small job that needs to run right after the current code.

More About Priority of Micro and Macro Tasks

In JavaScript, as we learned tasks within the event loop are categorized as either microtasks or macrotasks, each with a specific priority. Microtasks include operations like processing Promises, and they always have a higher priority. This means that after the current script finishes, the JavaScript engine will handle all pending microtasks before starting on any macrotasks, like timeouts or event handlers.

Here’s a simple example to see this in action:

console.log('Start'); setTimeout(() => console.log('Timeout 1'), 0); // Macrotask Promise.resolve().then(() => { console.log('Promise 1'); // Microtask Promise.resolve().then(() => console.log('Promise 2')); // Chained Microtask }); setTimeout(() => console.log('Timeout 2'), 0); // Macrotask console.log('End');
Expected output:
  • "Start"
  • "End"
  • "Promise 1"
  • "Promise 2"
  • "Timeout 1"
  • "Timeout 2"

This sequence shows how microtasks (Promise resolutions) are executed immediately after synchronous code, even before macrotasks scheduled for the same time. This prioritization ensures that critical tasks like updating data are completed as soon as possible.


Understanding the Event Loop, along with microtasks and macrotasks, can greatly enhance your ability to write efficient JavaScript. By knowing how and when JavaScript executes your code, you can create more responsive and efficient applications. The key takeaway is to use macrotasks for big, less urgent tasks, and microtasks for small, immediate jobs. This guide should help you grasp the basics and start applying these concepts in your own projects!

Practice Your Knowledge

In JavaScript, what happens when a promise resolves and there is a `.then()` handler attached?

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