What Specific JavaScript Optimization Techniques Can Reduce First Input Delay (FID) on My Website?


To reduce First Input Delay (FID) on your website, you need to implement JavaScript optimization techniques such as reducing the main thread work, deferring non-critical JavaScript, utilizing web workers, and optimizing third-party scripts. Here’s a detailed guide to improve your site's FID.

Reduce Main Thread Work

Minimize JavaScript Execution

Reduce the amount of JavaScript that needs to be executed during the initial load. This can be done by breaking up long tasks and utilizing code-splitting techniques to ensure the critical parts of your code are loaded first while less essential code is loaded asynchronously.

Code Splitting

Use code-splitting techniques to break down large JavaScript bundles into smaller, more manageable chunks. This allows for faster initial load times and reduced blocking of the main thread.

Example: Webpack’s SplitChunksPlugin can be used to implement code splitting [Webpack Code Splitting, 2021].

Defer Non-Critical JavaScript

Use Async and Defer Attributes

For non-critical scripts, use the <script async> or <script defer> attributes to ensure they don’t block the parsing of the HTML document.

Example: <script src="example.js" defer></script> ensures that the script is executed after the document is parsed.

Reference: [Defer Non-Critical JavaScript, 2023].

Lazy Load JavaScript

Lazy load JavaScript so that scripts are only loaded when they are needed. This can significantly improve the user experience by ensuring that the main thread is not blocked by scripts that are not immediately necessary.

Reference: [Lazy Loading, 2021].

Utilize Web Workers

Offload Tasks to Web Workers

Web Workers allow you to run scripts in the background without blocking the main thread. Use Web Workers for expensive computations or other heavy tasks to keep the main thread free for handling user interactions.

Example: You can use a web worker for data processing tasks that are computationally intensive. Here is a basic setup: var worker = new Worker('worker.js');

Reference: [Using Web Workers, 2022].

Optimize Third-Party Scripts

Reduce Third-Party Script Impact

Third-party scripts can significantly impact your FID by blocking the main thread. Audit your third-party scripts to ensure they are absolutely necessary. Load third-party scripts asynchronously whenever possible.

Reference: [Third-Party JavaScript, 2023].

Use Performance Budgets

Implement performance budgets to keep track of the size and performance impact of third-party scripts. This ensures these scripts do not significantly degrade the user experience.

Reference: [Performance Budgets, 2021].

Implement Efficient Loading Strategies

Preload Important Resources

Use the <link rel="preload"> tag to preload critical resources such as fonts, CSS, and important JavaScript files. This ensures that crucial resources are available quickly.

Example: <link rel="preload" href="important.js" as="script">

Reference: [Preload Critical Assets, 2022].