6 Best Strategies to Optimize JavaScript Performance

Minimalistic designs are the cookie cutter for fast web performance. 

They involve fewer elements, such as images, graphics, and animations. And as you know, smaller file sizes mean a lighter website that loads fast.

But let’s be honest—a minimalist website isn’t everyone’s cup of tea. 

Most of us want to create intricate and immersive online experiences filled with fascinating features and interactive components. 

When it comes to creating a dynamic and interactive website, JavaScript comes in handy. 

But here’s the catch: 

JavaScript can cause serious performance issues on your website when not properly handled.

Unoptimized JavaScript files can delay rendering essential content on a web page and cause unexpected layout shifts. 

The results? A slowly loading page, a clunky and downright frustrating browsing experience for users.

Considering that JavaScript is among the heaviest files of a web page, it can be challenging to strike a balance between aesthetics and performance. But that doesn’t mean you can’t.

In this guide, we will show you how to optimize your JavaScript files for performance to provide an exceptional user experience on your website.

Optimize JavaScript Files through Minification

Usually, when writing code using your code editor, you use spacing, comments, and well-named variables to make code and markup readable.

It also helps others who might later work on the assets or your team you’re collaborating with understand your code.

However, when it comes to performance, these characters are useless to a machine. A web browser doesn’t need to read your comments in the code or recognize whitespaces to parse web page content.

Enter minification.

Minification removes unnecessary characters like whitespace, line breaks, comments, and redundant code from a source code without affecting its functionality. This reduces the overall size of your document leading to faster website performance. 

Consider this Javascript code which is not minified.

As you can tell, white spaces, line breaks, and comments have been generously utilized to make the code readable.

However, when it comes to performance, these characters don’t add value as a web browser perfectly ignores them.

Now here’s the exact version of the code after minification.

As you can see, it’s more compact, only occupying a single line of code. 

By removing unnecessary characters, you reduce the size of your JavaScript file, which can be transmitted more quickly over the network.

Although minification is an ideal way to optimize your JS scripts, doing it manually can be cumbersome. 

If you’re using WordPress as your CMS, JS minification is even simpler. Most caching plugins offer this feature. One good example is the WP Rocket. 

All you need to do is to mark the “Minify JavaScript files” check box and the plugin will automatically handle everything for you. 

If you’re not using WordPress, you can use a tool like UglifyJS to minify your JavaScript files.

JavaScript Bundling to Reduce HTTP Requests

JavaScript bundling is an optimization technique used to combine multiple JavaScript files into a single file, referred to as a bundle. 

This approach aims to reduce the overhead of establishing multiple connections, leading to faster page load times and improved performance. 

By minimizing the number of HTTP requests, bundling reduces network latency and decreases bandwidth consumption, resulting in a more responsive website.

To better grasp how bundling contributes to JavaScript performance enhancement, consider the following JavaScript files on our e-commerce website.

The cart.js file.

The product.js file. 

And the app.js file. 

Without bundling, the browser would need to make separate network requests to fetch each JavaScript file. 

In our case, it would make three requests for cart.js, product.js, and app.js. This can lead to increased latency and slower page load times, especially if the website has multiple JavaScript files.

Using a bundling tool like Webpack or Rollup, you can combine these files into a single bundled file. The result may look something like this:

As you can see, the bundled file contains all the necessary code from the original files in a single, self-contained unit.

Instead of making three separate network requests, the browser now only needs to fetch the bundle.js file.

Code Splitting

Code splitting involves splitting a large JavaScript bundle into smaller, more manageable chunks or modules that can be loaded on demand or in parallel.

By doing so, the browser can download and parse only the necessary code for the current page rather than loading the entire codebase. 

This reduces the initial load time and improves the perceived performance of the website.

For example:

Consider a JavaScript code snippet that triggers a popup modal when a user clicks a button on a web page.

In this case, we have two modules: modal.js and button.js. 

When the user clicks on the button with the ID myButton, it triggers the openModal() function from the modal.js module, which opens the modal.

Now, let’s discuss how code splitting can help improve the performance of this code.

In a non-split scenario, both the modal.js and button.js modules would be bundled together. However, as your codebase grows, the initial bundle size will increase.

When a user visits the web page, their browser has to download the entire bundle, which includes code related to the modal functionality, even if they never interact with the button.

To address this issue, we can apply code splitting to separate the modal-related code into distinct chunks. 

This approach reduces the size of the initial bundle, containing only the necessary code to render the button. Consequently, the initial load time is improved.

With code splitting, the modal-related code is loaded lazily only when needed. When the user clicks the button, the chunk containing the openModal() function is fetched asynchronously.

As a result, the modal code is not downloaded and executed until the user interacts with the button. This optimization significantly enhances the initial load time, as the browser doesn’t have to fetch and execute the modal code immediately.

You can utilize tools such as Webpack or Rollup to implement code splitting. These bundlers analyze the dependencies and split the code into separate chunks based on the configuration settings you provide.

When visitors land on your website, their web browser begins reading your site’s code from the top and progresses downwards. 

During this process, if the browser encounters a “<script>” tag in HTML, it pauses the parsing of the HTML document, fetches and executes the JavaScript file, and then resumes parsing.

Unfortunately, this synchronous behavior brings about certain drawbacks. 

The browser has to wait for the JavaScript file to be completely downloaded and executed, causing delays. 

Consequently, the HTML content that appears above the fold will take longer to display, creating the perception of a slower website for your visitors.

Just think about it—the time spent in this “paused” state, downloading and parsing those resources, could be utilized more productively. 

For instance, it could be used to load the visible portion of your website’s content that immediately captures the attention of someone landing on the page.

Enter the “defer” and “async” attributes in JavaScript.

These two attributes help fix the render-blocking behavior of JavaScript, where the page would have to load and execute scripts before finishing rendering the page.

Let’s learn a little more about the two JavaScript attributes.

Defer Attribute:

When you add the “defer” attribute to the “<script>” tag, you tell the browser to continue parsing the HTML document while fetching the JavaScript file in the background.

This means the browser will only execute the JavaScript code once the HTML parsing is complete.

This ensures that scripts are run in the order they appear in the HTML and do not obstruct the parser.

Take a look at this code, for example:

Adding the “defer” attribute tells the browser to fetch the “script.js” file asynchronously while parsing the HTML document. 

And when the HTML parsing is complete, execute the JavaScript code in “script.js”.

Async Attribute:

The async attribute indicates that the script should be executed asynchronously as soon as it is downloaded. 

Here’s how it works:

When a browser encounters a script with the async attribute, it immediately fetches it without blocking HTML parsing.

Once the script is downloaded, it is executed immediately, even if the HTML parsing is not completed.

It’s, however, important to note that the async attribute is not suitable for all scenarios.

If a script has dependencies on other scripts or relies on certain elements of the web page being available, using the async attribute alone may lead to errors or unexpected behavior. 

In such cases, use the defer attribute to ensure proper script execution order.

If you’re using WordPress, you can optimize your JavaScript files using asynchronous loading without breaking a sweat. 

WP Rocket allows you to do that without touching a line of code on your website.

Here’s how to do it:

Once you’ve installed and configured the WP Rocket plugin, proceed to the “File Optimization” tab.

Locate the “Load JavaScript deferred” option and mark the checkbox. 

Simple as that.

Use a Content Delivery Network

Using a content delivery network (CDN) can significantly optimize the delivery and performance of your JavaScript files.

CDNs consist of a network of servers distributed across multiple geographic locations. 

When you host your JavaScript files on a CDN, these files are replicated and stored on servers closer to your website visitors. 

This geographical proximity reduces the distance data needs to travel, resulting in faster response times and improved performance.

Here’s a list of the best global CDN providers to reduce latency and improve page load times.

• Cloudflare
• Stakepath
• RocketCDN
• Fastly
• Sucuri
• KeyCDN 

Using the HTTP/2 Protocol

The HTTP/2 protocol is another method to optimize JavaScript files and improve the performance of your website.

The new version of the transfer protocol provides various significant features designed to enhance web communication efficiency. 

One of these features is multiplexing. This feature allows multiple requests to be sent and received simultaneously over a single TCP connection. 

This eliminates the need for establishing and maintaining multiple connections, resulting in reduced latency and accelerated delivery of JavaScript files.

Another key functionality is header compression. Unlike HTTP/1.1, where headers are sent with each request and response, HTTP/2 employs a more efficient compression algorithm. 

This compression effectively reduces the size of headers and minimizes the amount of data transferred, leading to improved performance.

And the cherry on top—the server push. 

This remarkable feature allows the server to be proactive and send resources to the client even before they’re requested. 

This means loading your JavaScript files without additional client requests, reducing round-trip time, and minimizing latency associated with fetching resources.

Improve JavaScript Performance

JavaScript is like a magical wand that brings life to your website. It enables you to build engaging user interfaces that respond to user actions, providing a rich and interactive experience. 

However, all these additions come at a cost. They consume your users’ precious resources, from CPU and GPU to memory and network capabilities.

Optimizing JavaScript files allows you to improve the overall performance of your website. By minimizing the file size and reducing the number of HTTP requests, you can significantly enhance the loading speed and user experience of your site or web applications.