Event Loop and Concurrency Model in JavaScript

by Hexagon, 2 minutes read javascript guide

In this article, we delve into the Event Loop and Concurrency Model in JavaScript, revealing how they enable non-blocking asynchronous behavior.


JavaScript is often assumed to be a single-threaded language, which means it executes one operation at a time in a single sequence, or thread. However, thanks to the Event Loop and Concurrency Model, JavaScript can handle many tasks like UI updates, API calls, and more, all at the same time. Understanding these mechanisms is crucial for writing efficient and fast code.

Concurrency Model

The Concurrency Model in JavaScript is based on an 'event loop,' a mechanism that enables non-blocking behavior. Here's a breakdown of the main components:

The Call Stack

JavaScript uses a single call stack to manage function execution. When a function is called, it's added to the stack. When it returns, it's removed from the stack.


The heap is where objects are stored. Variables and function declarations take up space in memory and are stored in the heap.

Web APIs and External Resources

JavaScript uses Web APIs provided by the browser for tasks like DOM manipulation, fetching data, and timeouts. These are not part of the JavaScript runtime itself but interact with it.

The Event Loop

The Event Loop has one simple job: to monitor the Call Stack and the Callback Queue. If the Call Stack is empty, it takes the first event from the queue and pushes it to the Call Stack, which effectively runs it. This process is what allows JavaScript to perform non-blocking operations.

Microtasks and Macrotasks

JavaScript tasks can be categorized into Microtasks and Macrotasks. Microtasks are processed immediately after the current script is executed and include promises and DOM mutations. Macrotasks include setTimeout, setInterval, and user interface updates.

Callback Queue

When an asynchronous operation is complete or a timer counts down, the corresponding callback or event handler is added to the Callback Queue. The Event Loop checks this queue and pushes the callback to the Call Stack when it's empty.

Single-threaded vs. Multi-threaded

JavaScript is single-threaded, but its non-blocking nature makes it capable of handling multiple operations concurrently via the Event Loop.

Asynchronous vs. Concurrent

While JavaScript is not inherently asynchronous, it gains asynchronous capabilities through callbacks, promises, and async/await, thanks to the Event Loop.

Anti-Patterns to Avoid

  1. Blocking the Event Loop: CPU-intensive tasks can block the Event Loop, making the UI unresponsive.
  2. Nested Callbacks: Often called 'Callback Hell,' can make code hard to read and maintain.

Resolving Blocking the Event Loop

Bad Practice

// CPU-intensive task that blocks the Event Loop
for (let i = 0; i < 1e9; i++) {
  // Do something here

Good Practice

// Using Web Workers to offload tasks and avoid blocking the Event Loop
// This is runtime dependent, hence omitted

Resolving Nested Callbacks

This is mentioned in various ways earlier in the series, but worth repeating:

Bad Practice

getData(function (a) {
  parseData(a, function (b) {
    validateData(b, function (c) {
      // Deeply nested structure

Good Practice

// Using Promises
// Using Async/Await


Understanding the Event Loop and Concurrency Model in JavaScript is essential for writing efficient, non-blocking code. This knowledge is foundational for anyone looking to become a proficient JavaScript developer.

Design Patterns