The JavaScript Checker runs your scripts, reports console errors and warnings, flags render-blocking patterns, measures bundle size and parse time, and tests how Googlebot's headless Chromium sees your page. JS errors silently break features, hurt Core Web Vitals, and prevent crawler indexing. This index covers every category of finding.
Pick the category matching your finding:
<script src> in <head> without async or defer halts page render. Add defer for scripts that need DOM, async for independent ones (analytics), inline only critical-path. The impact on Largest Contentful Paint.requestIdleCallback or setTimeout, move heavy computation to Web Workers, virtualise long lists.Common JS findings vary by stack:
The checker loads your page in headless Chromium, captures every console error and warning, measures bundle size and parse/execute time, identifies render-blocking patterns, lists third-party tags with their byte and time cost, and replicates how Googlebot sees the rendered DOM. For the full reference, see the JS Checker Guide or sample report.
Run the checker. Most sites have 3-5 specific scripts costing them seconds of load time — kill those and pages feel instantly faster.
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