JavaScript Defer Checker

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JavaScript Defer Checker: Review Script Loading Attributes

Scripts can block HTML parsing while they load and execute. The defer and async attributes change that behavior. This checker finds script tags and reports which attributes they use.

How the Attributes Work

Normal script: the browser stops parsing, fetches the script, and runs it immediately. This blocks rendering until the script is done.
async: the script is fetched in parallel and runs as soon as it is ready, which may be before or after the page finishes parsing. Execution order is not guaranteed.
defer: the script is fetched in parallel and runs after the HTML is fully parsed, in the order it appears. Execution order is preserved.

Which Attribute to Use

defer for scripts that depend on the DOM or on other scripts, and for most site scripts.
async for independent scripts such as analytics or ads that do not rely on other scripts or the DOM.
Neither only when the script must run immediately, which is rare and should be deliberate.

Why It Matters

Render blocking: normal scripts in the <head> delay the first paint.
User experience: faster rendering means the page becomes usable sooner.
Performance metrics: reducing blocking scripts improves loading and interaction timings.

Common Problems

Site-wide scripts in the <head> without defer or async.
Async applied to scripts that depend on each other, causing order-dependent bugs.
Defer applied to inline scripts, where it has no effect.
Third-party scripts added by plugins that do not control loading attributes.

Practical Uses

Auditing a page for render-blocking scripts.
Reviewing script loading after a performance change.
Checking a new theme or template.
Finding third-party scripts that need adjusting.

Limitations

The checker reports the attributes present on script tags. It does not measure actual loading impact, which depends on script size, network conditions, and execution time. Attribute presence is a starting point for performance review.

Summary

Defer and async prevent scripts from blocking rendering when used correctly. This checker shows how each script is loaded, helping you identify blocking scripts and choose the right attribute.