How Can I Improve JavaScript Execution Time by Breaking Up Long Tasks and Optimizing Event Handlers for Better FID Scores?


Improving JavaScript execution time can be achieved by breaking up long tasks into manageable chunks and optimizing event handlers. These techniques contribute to better First Input Delay (FID) scores, a key performance metric for web page responsiveness. The process encompasses task scheduling, web workers to handle heavy computations, passive event listeners, debouncing and throttling techniques.

Breaking Up Long Tasks

Task Scheduling

JavaScript is a single-threaded language, meaning it can only do one thing at a time. If a executing long task, it can block the user interface, causing an unpleasant user experience. You can use task scheduling to divide long tasks into smaller tasks. One useful method is the `setTimeout` function, which pushes code to the event loop and allows the browser to keep the interface responsive [MDN Web Docs, 2022].

Web Workers

Web workers provide a means to run JavaScript on background threads, allowing for heavy computations to be processed apart from the main thread, therefore not blocking the user interface. The computation will run concurrently with all other scripts running in the main task queue [MDN Web Docs, 2022].

Optimizing Event Handlers

Passive Event Listeners

Passive Event Listeners are a performance feature that can boost scroll performance in certain types of event listeners. Setting the `passive` option on your event listener to `true` improves scrolling performance by telling the browser that no preventDefault will be called, and therefore not needing to wait for JavaScript [Chrome Platform Status, 2016].

Debouncing and Throttling

Debouncing and Throttling are techniques to limit the number of times an event handler is invoked, improving performance. While similar, they serve different purposes. Debouncing allows an event to trigger after a certain amount of time has passed without the event being invoked. Throttling limits the number of times an event can be triggered during a specified time period [CSS-Tricks, 2017].


Improving JavaScript's execution time and enhancing FID scores involves a combination of breaking tasks into manageable pieces and using optimization techniques for event listeners. By implementing task scheduling, web workers, passive event listeners, and debouncing/throttling techniques, you can effectively improve your JavaScript execution time and create a more responsive, user-friendly interface.