Encyclopedia / JavaScript
JavaScript
The programming language of the browser.
It reads and changes the DOM, responds to events and talks to servers. Every kilobyte has to be downloaded, parsed and run, often on a slow phone, so the best-performing sites ship as little as possible and keep content readable without it.
Related terms
Where it shows up in a page load
- Step 10: JavaScript makes it interactive. Scripts run, attach event listeners and may fetch more data from an API. How quickly the page responds to a tap is INP, another Core Web Vital.
Watch the full request journey →
More in JavaScript
What happens when someone clicks, types or scrolls.