What Is JavaScript and What Is It Used For?

What it applies to

Any interactive behaviour on a website — form validation, animations, API calls, dynamic content updates, and complex UI interactions that HTML and CSS alone cannot achieve.

How JavaScript runs on a webpage

• JavaScript files are loaded in the browser via <script> tags in the HTML — either inline or as external .js files.
• The browser parses and executes JavaScript on the main thread. Heavy scripts block rendering — which is why render-blocking JavaScript is a primary cause of slow page load.
• Use the defer attribute on non-critical scripts to load them after HTML parsing: <script src="app.js" defer></script>
• JavaScript manipulates the DOM — the browser's in-memory representation of the HTML — to add, remove, or modify elements dynamically without a page reload.

In Webflow

Webflow handles most interactive behaviour (animations, interactions, CMS filtering) without custom JavaScript. Add custom JS via Project Settings → Custom Code → Footer Code, or per-page via Page Settings. For complex integrations, use <script> tags via Webflow's Embed element, keeping custom code minimal and well-commented.

Performance rule

Audit third-party JavaScript regularly using Chrome DevTools → Coverage. Unused JavaScript loaded on every page is one of the most common causes of poor Core Web Vitals scores.