Technical SEO Audit Checklist for JavaScript Websites

JavaScript-heavy sites fail technical SEO audits in ways HTML sites simply don't: content, links, and schema that exist perfectly well in the browser can be invisible to a crawler if they depend on client-side execution that never completes the way a human's browser does. Most ranking issues we see trace back to crawlability or indexation — before touching content or keywords, that's the layer to check first.

Start with what's actually in the initial HTML

View-source (not the rendered DOM in DevTools) on your key pages. If your primary heading, body copy, and internal links aren't in that raw response, you're relying on the crawler to execute JavaScript correctly — which happens, but on a delay, and not identically across every crawler that matters.

Rendering delays are real and vary by crawler

Google renders JavaScript in a second wave after initial crawling, which can mean indexing delays measured in days rather than minutes for pages that depend entirely on client-side rendering. Other crawlers — including the ones AI search tools use — vary even more widely in how much JavaScript they execute. For anything time-sensitive (new products, price changes, breaking content), that lag directly costs visibility.

Core Web Vitals thresholds to check against

Confirm real, field-measured performance against current thresholds, not just a lab score:

• Largest Contentful Paint (LCP): under 2.5 seconds
• Interaction to Next Paint (INP): under 200 milliseconds
• Cumulative Layout Shift (CLS): under 0.1

Canonical tags are still widely broken

Canonical adoption across the web sits at roughly two-thirds of sites — meaning a large share of pages are silently sending authority to the wrong URL through a missing or misconfigured canonical tag. On JavaScript sites specifically, canonicals injected client-side can be missed entirely if a crawler doesn't wait for them to render.

The full checklist

• Crawlability: robots.txt isn't accidentally blocking JS/CSS or key routes
• Indexation: server or static rendering enabled on every route meant to be indexed
• Core Web Vitals: LCP, INP, and CLS measured from real field data, not just lab tools
• Canonicals: self-referencing on every indexable page, correctly resolved after rendering
• Redirects: no unnecessary chains, no client-side-only redirects search engines might miss
• Structured data: present in the initial HTML or reliably rendered, matching visible content
• Log file analysis: confirm which pages crawlers actually visit versus which you assume they do
• Migration protection: redirect maps and rendering parity checked before and after any rebuild