In more detail
Many modern sites are built with JavaScript frameworks such as React, Vue or Angular. Some send nearly empty HTML and build the page in the browser (client-side rendering). Others prepare the HTML on the server first (server-side rendering or static generation), then add interactivity.
Google's JavaScript SEO basics explains that it processes JavaScript pages in three phases: crawling, rendering and indexing. Pages may wait in a queue before Google renders them with a recent version of Chrome. Content that only appears after rendering can still be indexed, but problems at this stage are a common cause of missing pages.
Typical issues include:
- Links built with click handlers instead of real
<a href>elements, so crawlers can't follow them - Important text loaded only after user interaction, such as clicking a tab
- Scripts or API calls blocked by robots.txt
- Titles, canonical tags or noindex directives changed by JavaScript after load
- Heavy scripts hurting Core Web Vitals, especially responsiveness
Other search engines and AI crawlers may render JavaScript less reliably than Google, which is another reason to serve key content in the initial HTML where possible.
Why it matters when hiring an agency
If your site uses a JavaScript framework, ask agencies about their experience with it specifically. They should be comfortable comparing the raw HTML with the rendered page, using the URL Inspection tool in Search Console, and talking to your developers about rendering options.
JavaScript problems usually need developer time to fix, so factor that into budgets and timelines. An agency that only lists issues without explaining the fix in developer terms will slow things down.