JavaScript defer checker

What is a JavaScript Defer Checker?

The JavaScript Defer Checker lists every script tag on a page and reports whether each one uses the defer or async loading attribute, or neither (render-blocking).

Defer vs Async vs Neither

• Neither (default) — The browser pauses HTML parsing entirely to fetch and execute the script, directly delaying when the page becomes visible.
• defer — The script downloads in parallel with parsing but executes only after the document is fully parsed, in the original order — ideal for scripts that depend on the DOM.
• async — The script downloads in parallel and executes as soon as it's ready, in whatever order that happens — best for independent scripts like analytics.

Render-blocking scripts are one of the most common causes of slow First Contentful Paint and Largest Contentful Paint scores.