Speeding Up Page Loading with JavaScript: Defer vs. Async
2 mins read Published On: Thu Jan 18 2024

In the world of web development, optimizing page loading speed is crucial for providing a smooth user experience. One area where this optimization comes into play is the handling of JavaScript files, which can be larger and heavier than HTML files. In this blog post, we’ll explore two techniques—defer and async—that can help improve your website’s performance.

The Challenge of JavaScript Files

JavaScript files are inherently bigger and heavier than HTML files. When a browser encounters a <script> tag, it pauses everything else and starts executing the script. This can lead to slower download speeds and increased processing time, affecting the overall responsiveness of your web page.

Option 1: Using defer

One way to address this issue is by using the defer attribute in your <script> tags. When you use defer, the browser downloads the script simultaneously while building the DOM. However, it doesn’t block the page, allowing the page to load without interruption. The script executes when the DOM is fully built but before the DOMContentLoaded event fires.

Pro Tip: defer is best suited for external scripts and preserves the order of script execution as specified in the HTML.

Option 2: Using async

Another option is to use the async attribute in your <script> tags. Like defer, async also enables the browser to download scripts while building the DOM. However, there are two key differences:

  1. Scripts with async run as soon as they are downloaded, without waiting for the DOM to finish building.
  2. The order of script execution is not preserved; scripts execute in the order they are downloaded.

Pro Tip: Similar to defer, async is suitable for external scripts. Use it for scripts that are not dependent on the DOM’s structure.

Bonus: Dynamically Loading Scripts

For more control over script loading, consider dynamically adding scripts to your web page using the createElement('script') method. By default, dynamically loaded scripts behave like async scripts, starting to load as soon as they are appended to the document. If you need a script to behave like a defer script, simply set script.async to false.

In conclusion, optimizing the loading of JavaScript files is essential for faster web page performance. Choose between defer and async based on your script’s dependencies and execution requirements. And don’t forget the option of dynamically loading scripts when you need precise control.

By implementing these techniques, you can enhance your website’s speed and provide a seamless user experience for your visitors.