How To Improve Website Performance By JavaScript Optimization

How To Improve Website Performance By JavaScript Optimization

Considering the use of JavaScript on your website and about mitigating performance issues.

Byte for byte, no resource affects page speed more than JavaScript. JavaScript affects network performance, CPU processing time, memory usage, and overall user experience. Inefficient scripts can slow down your website, making it less responsive and more frustrating for users. Furthermore, while images and video account for over 70% of the bytes downloaded for the average website, JavaScript has a greater potential for negative performance impact, can significantly impact download times, rendering performance, and CPU and battery usage.

Optimizing JavaScript performance is essential for delivering fast, responsive, and efficient web applications. By following best practices, leveraging modern tools, and incorporating advanced techniques, developers can significantly improve JavaScript performance.

Why Optimize JavaScript?

JavaScript can significantly impact the performance of your web application. Poorly optimized JavaScript can slow down load times, increase rendering times, and consume more CPU and battery resources. Optimizing JavaScript ensure that your web application runs smoothly, providing a better user experience and potentially increasing user engagement.

While some of the techniques for optimizing your JavaScript code for better performance include minification and compression, code splitting, lazy loading, avoiding blocking scripts, caching strategies, reducing DOM manipulations, and optimizing loops & conditions.

Some of the JavaScript Optimization tips from Google:

1. Avoid JavaScript File Proliferation

The number of JavaScript files could become excessive if not careful, especially when a UI component is in a separate file. Thus, avoid proliferation in the number of JavaScript files on your site. Further, reducing the number of JavaScript files that the browser downloads helps in improving website performance.

The opportunity section of the PageSpeed Insights report lists recommendations specific to the site. Go through those recommendations and keep request counts low and transfer sizes small. One of the ways of fixing the problem that Google has recommended is to combine smaller files together for having single, large file to download. Another way is to support HTTP2 on your site, improving performance without joining files.  

2. Avoid Excessive DNS Lookups

Avoiding an excessive number of DNS lookups for reference JavaScript files slows down the user’s first visit to the site. PageSpeed Insights shows a list of domain names used in URLs in sections like reducing JavaScript execution time. Using the Network tab in Chrome Developer Tools helps you see all the domain names referenced. Consider hosting a copy of externally referenced JavaScript files on your own site to reduce the number of DNS lookups.

3. Eliminate Inefficient JavaScript

Reducing or eliminating inefficient JavaScript can slow down webpages, leading to poor user experiences. Opportunities in Google’s PageSpeed Insights report include reducing JavaScript execution time, eliminating render-blocking resources, document.write, and not using passive listeners. For writing JavaScript code differently, techniques such as profiling existing code and writing one's own scaled-down versions of more powerful components can be employed.

4. Eliminate Unused JavaScript

There is a problem of inefficient unused JavaScript, as reusing code across sites can lead to the inclusion of JavaScript that is not needed. Even if JavaScript is never used, the web browser must download and parse it, which wastes resources. Google recommends a technique called tree-shaking that can be used to identify what is never called, which is safe to delete.

5. Compress JavaScript Files 

JavaScript files are compressed when downloaded, while the web browser has to spend more CPU time decompressing the file content. JavaScript files benefit from being compressed. By clicking enable text compression showing you which files are recommended to be compressed. Most web browsers or content management systems have built-in support for compressing downloads if properly configured.

6. Set Appropriate Cache Durations for JavaScript Code

JavaScript files are returned with appropriate cache expiry time headers, which helps browsers to avoid the overhead of checking in case of an outdated cache of JavaScript files. This ultimately improves performance. Chrome Developer Tools’ Networking tab allows you to view HTTP response headers for downloaded JavaScript scripts. Looking for headers like Cache Control. In PageSpeed Insights, look for the opportunity titles “Serve static assets with an efficient cache policy.” Clicking on it will provide a list of resources, including JavaScript files, that could benefit from property-configured cache headers.

Enhancing the caching of commonly used JavaScript files includes referencing files from a shared public location. When a user visits sites that reuse the same JavaScript file, enabling the browser to use the previously downloaded copy of the file which would improve performance.