Altcademy - a Forbes magazine logo Best Coding Bootcamp 2023

What is lazy loading in JavaScript

Understanding the Concept of Lazy Loading

Lazy loading is a design pattern commonly used in computer programming to defer initialization of an object until the point at which it is needed. It can contribute to efficiency in the program's operation if properly and appropriately used. The opposite of lazy loading is eager loading.

To understand lazy loading, imagine you're at a buffet. You have a plate, and there's a whole array of dishes. Eager loading is like grabbing every single dish and putting it on your plate right from the start, while lazy loading is like only getting a dish when you actually want to eat it.

Why Use Lazy Loading?

In the buffet analogy, eager loading might seem wasteful. It's the same in programming. Loading everything can be time-consuming and resource-intensive, especially when dealing with a large amount of data. With lazy loading, we reduce the initial load time and save resources because we're only loading what's necessary.

Lazy Loading in JavaScript

In JavaScript, we can implement lazy loading in various ways. However, for this article, we'll focus on lazy loading images and JavaScript modules.

Lazy Loading Images

When building a web page with many images, loading all images at once can slow down the page. With lazy loading, we can load only the images within the user's viewport and defer the loading of other images until the user scrolls to them.

Here's a simple way of implementing lazy loading for images using JavaScript:

document.addEventListener("DOMContentLoaded", function() {
  let lazyImages = []"img.lazy"));

  if ("IntersectionObserver" in window) {
    let lazyImageObserver = new IntersectionObserver(function(entries, observer) {
      entries.forEach(function(entry) {
        if (entry.isIntersecting) {
          let lazyImage =;
          lazyImage.src = lazyImage.dataset.src;

    lazyImages.forEach(function(lazyImage) {

In this code, we use the IntersectionObserver API to observe each image with the class "lazy". When an image enters the viewport (isIntersecting), we replace its src with the actual image URL (dataset.src), remove the "lazy" class, and stop observing it (unobserve).

Lazy Loading JavaScript Modules

JavaScript also allows for lazy loading of modules. This can be useful for large web apps, where you may not want to load all JavaScript at once.

Here's an example:

button.addEventListener('click', event => {
  .then(dialogBox => {;
  .catch(error => {
    /* Error handling */

In this code, we use the dynamic import() function to load a JavaScript module when a button is clicked. The import() function returns a promise that resolves into a module object. From there, we can use any exports from the module.

Benefits and Drawbacks of Lazy Loading

Lazy loading has its pros and cons. On the plus side, it can significantly improve load times, enhance user experience, and save resources. On the downside, it can cause a delay in content appearance, which might affect user experience if not handled gracefully.

It's like going back to the buffet. Lazy loading allows you to enjoy your meal (the web page) without feeling overwhelmed, but if the kitchen (server) is slow, you might wait for a while before getting the dish you want.

Let's Be Lazy (But in a Good Way)

In conclusion, lazy loading is a powerful technique that, when used correctly, can improve the performance and user experience of your web app. It's about being smart and efficient - grab what you need when you need it, rather than all at once. Remember, however, that each situation is different and requires careful consideration. So, keep exploring, keep learning, and keep innovating. Happy coding!