A single slow-loading page can make or break a customer relationship.
Studies have shown that even a 0.1-second load time shift can influence every aspect of the user journey. The list of tasks to make your website faster seems never-ending.
But you already knew that.
Whether dealing with image-heavy websites, complex single-page applications, or content-rich platforms, JavaScript lazy loading offers a flexible solution to optimize your site's performance.
So, in the following lines, you will discover everything you need to know about JavaScript lazy loading. We’ll cover what it is, how it works, and how to implement it to boost your website’s performance.
Although implementing JavaScript lazy loading can be challenging, with NitroPack, you can sit back and relax as it automatically handles this task.
So, what exactly is lazy loading?
Essentially, it is a clever technique that prioritizes loading essential content first, leading to a more responsive website. JavaScript lazy loading defers the loading and execution of JavaScript files and elements until needed.
It loads resources only when necessary, usually when the user scrolls to a specific part of the webpage or interacts with a particular element.
Specific actions can trigger lazy loading when a user navigates through a webpage. For instance, scrolling down a page can prompt the loading of images, videos, or JavaScript-driven elements that are not immediately visible when the page first loads.
The "fold" refers to the portion of a webpage that is visible without scrolling. Content above the fold loads immediately, while content below the fold can be lazy-loaded.
This approach ensures that users see the essential parts of the page quickly, improving the site's perceived performance. By loading this essential content upfront, you'll score a quick First Contentful Paint (FCP), leading to a better user experience. In the background, JS lazy loading will help you ensure your Largest Contentful Paint (LCP) is also optimized, contributing to a passing grade on Core Web Vitals.
Let's break this down a bit more. Lazy loading doesn’t involve just images or basic content; it's also about:
JavaScript lazy loading is a powerful tool that can boost your website's performance.
But it's not a one-size-fits-all solution. Let's explore the good, the bad, and the "hmm, that's something to think about" aspects of JavaScript lazy loading.
Discover the benefits that make JavaScript lazy loading a great choice.
Implementing lazy loading for JavaScript resources can initially seem like getting lost in a maze, but don't worry—we're here to simplify things.
Below are a few ways to implement Javascript lazy loading. Try them and see what works best for you.
Code splitting involves breaking your JavaScript into smaller, more manageable chunks. This ensures that only the code required for the current page is loaded initially, with additional code being loaded as needed. This approach can greatly reduce the initial load time of your web pages.
Steps for code splitting:
Asynchronous loading allows JavaScript files to load without blocking the rendering of the rest of the page.
Steps for asynchronous loading:
Dynamic imports enable you to load JavaScript modules only when they are needed. This is particularly useful for loading features or components not immediately required when the page loads.
Steps for dynamic imports:
The Intersection Observer API is a powerful tool for lazy loading. It lets you detect when elements (like images or JavaScript resources) enter the viewport. When an element comes into view, the corresponding script can be loaded, ensuring resources are only loaded when necessary.
The observer API is a pretty complex beast, so we strongly recommend referring to the Mozdev documentation for examples and explanations of how to implement it. With this in mind, here is an example of implementation for the Observer API
When the element with the class lazy-load-script enters the viewport, the script specified in the data-src attribute is dynamically created and appended to the body, thus being loaded. This method can be applied to any existing HTML element, not just empty divs, providing flexibility in how lazy-loaded scripts are integrated into your page.
Steps for implementing the intersection observer:
This strategy involves loading resources as the user scrolls down the page. For example, if a user scrolls to a specific section, you can trigger the loading of the JavaScript code associated with that section. This is useful for loading content that's not immediately visible but will be needed as the user explores the page.
Steps for lazy loading elements on scroll events:
Instead of loading all images when a page initially loads, lazy loading defers the loading of off-screen images until the user scrolls near them. While many open-source scripts and code libraries, like Lozad.js and Vanilla Lazyloadd, handle this, understanding the underlying logic of lazy loading JS for images is beneficial.
Here are some key points on how to effectively lazy load images:
Example of lazy loading JS for images:
Instead of constantly updating your code manually or hiring a web developer to do it, you can install NitroPack and lazy load your resources in a couple of minutes.
NitroPack's Delayed Scripts feature allows you to lazy load your JS scripts:
All you have to do is find the script you want to delay, copy the first line and paste it into NitroPack:
Then, NitroPack will execute the script with a delay of about 6500ms, thus giving loading priority to your other scripts, which are much more important for the user experience.
Simply put, it's an efficient and easy way to get the most out of JS lazy loading without the manual hassle.
Before you jump into JavaScript lazy loading, let's ensure it's your best solution. You could start with simpler approaches like optimizing image sizes, minifying code, and leveraging caching. If lazy loading is still your choice, here are some best practices for a smooth setup.
Before diving into lazy loading, ensure your images and videos are optimized for quick loading. Consider the following steps:
Using little placeholder pictures or low-quality versions of your images is also a good idea. This keeps your page from jumping around as the real images load, which is crucial for user experience and SEO. These placeholders let users know the content is loading so they know what to expect.
Consider using the Intersection Observer API. It's a great way to detect when an element comes into view, making it perfect for lazy loading. It's more efficient than older techniques involving event listeners for scroll events, as it’s specifically designed for this purpose.
Remember to test your lazy loading implementation across different devices and connections. Make sure your site looks good on phones, tablets, and older browsers. Check how it loads on slow connections, too, including 3G. You want to ensure everyone has a good experience.
Ensure search engines can crawl and index your lazy-loaded content. Follow Google's guidelines for lazy loading and use SEO tools to check if everything is working as expected. You might need server-side or dynamic rendering for complex setups to help crawlers access your content.
Have a fallback plan for older browsers. Modern browsers support Intersection Observer and loading="lazy," but older ones might not. Load all content immediately for these browsers. Having a fallback ensures no users are left out, no matter their browser.
Use performance monitoring tools to measure the impact of lazy loading on your site. Tools like Google's Lighthouse, PageSpeed Insights, and WebPageTest can provide insights into how well your lazy loading implementation works.
Have we covered everything? We sure hope so. Before we wrap this up, here are some frequently asked questions that should address any lingering doubts.
Interactive components that require JavaScript can be lazy-loaded using the same logic as the images exemplified above using the Intersection Observer API. Simply set these components to load when they come into view to ensure that your page loads faster initially and only moves to load the necessary JavaScript when the user interacts with your expandable section or tabbed element.
Third-party widgets can and should be lazy-loaded as they consume many resources and come with heavy scripts that can destroy your initial page load. You can use a lazy-loading library or the intersection Observer API to load third-party widgets only when they come into the user’s viewport.
Third-party tracking scripts can also be deferred using lazy loading techniques. However, lazy loading requires more testing to ensure critical tracking information is not lost. We strongly recommend installing only the necessary tracking tools to avoid overloading your code.
Surprise, surprise: animations can also be lazy-loaded using the same logic of only being loaded when they are about to enter a viewport. This ensures that there is no delay in your initial load time. The best way to handle resource-intensive animations is by using the Intersection Observer API.
The most important thing is to check whether the browser supports lazy loading. Earlier, we advised providing a fallback that loads content immediately. This is useful for older browsers that do not support IntersectionObserver or the loading=lazy attribute.
Of course! NitroPack’s lazy loading feature does all of these optimizations automatically for you. Alternatively, you can use several tools or libraries like the ones mentioned above (Lozad.js, Vanilla Lazyload, etc.) to simplify the implementation process.
Niko has 5+ years of experience turning those “it’s too technical for me” topics into “I can’t believe I get it” content pieces. He specializes in dissecting nuanced topics like Core Web Vitals, web performance metrics, and site speed optimization techniques. When he’s taking a breather from researching his next content piece, you’ll find him deep into the latest performance news.