JavaScript Caching Test

Browser caching of JavaScript is one of the highest-leverage wins for content-heavy sites where users browse multiple pages per session. Cached scripts skip the network entirely on repeat visits, avoiding the full download and parse cost on every subsequent load while still letting you push fresh code through versioned filenames. This test checks whether the external JavaScript files served with your page include the caching headers that let browsers store and reuse them, and flags any responses that miss out on browser caching.

Why JavaScript caching matters

JavaScript bundles can be the largest single asset on a page beyond images, and they typically change far less often than the HTML or content. That makes them ideal for aggressive caching: a returning visitor with the script already cached avoids the entire download and parse cost on every subsequent page load. On a content-heavy site where users browse multiple pages per session, properly cached JavaScript dramatically improves perceived speed across the entire visit.

Modern best practice pairs long cache lifetimes with versioned filenames: every build emits files like app.7f3c1e.js, where the hash changes whenever the contents change. With this pattern you can set Cache-Control: public, max-age=31536000, immutable safely, because changes ship under new filenames and the cache can never serve stale code. Most modern build tools (Webpack, Vite, Rollup, esbuild) emit hashed filenames automatically.

Common caching gaps

• No Cache-Control header, forcing browsers to revalidate every request.
• Short max-age on versioned files where a year would be appropriate.
• no-cache or no-store directives on static JavaScript, which belong on dynamic responses instead.
• Cookies on script requests that prevent shared caching at the CDN.

This test reports the caching headers on your page's JavaScript files and flags responses that miss out on browser caching. The fix guide below covers configuring cache headers on the major web servers and CDNs, the role of build-time versioning, and how to verify caching is working as expected.