First identify what returned 404: the request may be for a JavaScript bundle such as /assets/app.js, or for an application page such as /dashboard opened directly or refreshed. A missing bundle calls for checking the build, asset URL, and deployed files. A missing client-side page usually calls for a host fallback to index.html. A route rewrite cannot restore a JavaScript file that is absent from the deployment.
1. Identify the exact request that failed
Open your browser’s developer tools and inspect the Network panel while reproducing the problem. Record the full request path, status, any redirects, and the response type. The distinction is important: a request ending in .js is an asset request; a path such as /dashboard may be a client-side route.
- If the page loads but a script request returns 404, troubleshoot the asset URL and deployed build.
- If in-app navigation works but opening or refreshing a page URL returns 404, troubleshoot server-side routing.
- If both fail, investigate each request separately; they may have different causes.
2. If a JavaScript bundle returns 404, verify the build and deployment
Confirm the file is in the production build
Run the project’s production build and inspect the generated output directory. Find the bundle named in the failed request. If it is not present in the build output, inspect the build configuration and the HTML or manifest that refers to it; a hosting rewrite will not create the missing file.
Make the host serve the right directory
Check the deployment platform’s configured publish or output directory against the directory actually produced by the build. The directory name depends on the framework and project configuration: dist is common, but it is not universal. Netlify’s guidance emphasizes matching the publish directory to the build output, and Vercel notes that a wrong output-directory setting can leave the platform serving an empty folder.
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After deployment, verify that the requested bundle exists in the files served by the active deployment, not just on your local machine. Compare the exact filename and path, including capitalization and directory segments.
Match the asset base path to where the app is hosted
An app deployed at a domain root and an app deployed under a repository or application subpath need not use the same asset URLs. Compare the script URL in the deployed HTML with the file’s location in the deployed output. If the app lives under a prefix, configure the framework’s public or base path to include it.
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For example, Vue CLI’s GitHub Pages guidance uses / as publicPath for a project hosted at https://<USERNAME>.github.io/<REPO>/. TanStack Router’s Vite deployment guidance likewise calls out matching Vite’s base setting to a subdirectory deployment. Vue CLI is in maintenance mode, so newer Vue projects should check the guidance for their current tooling.
3. If a page route returns 404, configure a client-side routing fallback
With history-based client-side routing, navigation can change the address in the browser without asking the server for a new document. But opening a route directly or refreshing it sends that route to the server. If the server has no matching file or route, it can return 404 even though the app’s in-page navigation works.
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Configure the hosting platform to serve index.html for application routes that should be handled by the client router. Scope the fallback so that real static assets and API endpoints continue to receive their normal handling. A broad catch-all can make an asset or API request receive HTML instead of the expected JavaScript or data.
- Netlify: Its documentation says history-based SPAs need a rewrite that serves
index.htmlfor requested URLs. The publish directory must also match the build output. - Vercel: Its guidance for Vite- or Create React App-style SPAs uses a catch-all rewrite to
/index.html, with API paths excluded where appropriate. This generic SPA rule is not suitable for every framework; use the framework-specific routing guidance when it applies. - Vue CLI: History-mode routing requires the production server to fall back to
index.html; settingpublicPathaddresses the deployment location of assets, not this server-routing requirement. - Azure Static Web Apps: Its routing configuration supports a fallback for client-side routes that do not correspond to deployed files.
- Cloudflare Workers static assets: SPA not-found behavior can serve
/index.htmlfor unmatched navigation requests. Review how navigation and API requests are handled before enabling that behavior.
4. Check for mismatched files after a release
If the 404 began after a deployment, inspect the deployed HTML and note the exact bundle filename it references. Compare that name with the files in the active deployment. Content-hashed bundles change names when their contents change; HTML that points to a hash not present in the active deployment will produce a missing-asset request.
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Netlify warns that code splitting or hashed filenames can cause references to break across atomic deploys when the HTML and assets being used do not align. Check the active deployment’s HTML and files before changing caching rules.
Consider browser and CDN caches separately
A cache can preserve an older page or asset reference. Azure Static Web Apps says its deployment cache invalidation is automatic, while a browser or separately configured CDN may still cache content. Check those layers if the failure differs between browsers, users, or cached and fresh sessions; do not assume that every host invalidates every external cache.
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5. Separate a custom-domain problem from a deployment problem
On Vercel, compare the deployment’s .vercel.app URL with the custom domain. If the deployment URL works and the custom domain does not, investigate domain and DNS configuration rather than assuming the bundle or application route is missing. On other hosts, use the equivalent platform-provided deployment URL where available; domain behavior and routing are platform-specific.
Quick Recap
Match the symptom to the next check
| Symptom | Likely area | Next check |
|---|---|---|
A .js bundle URL returns 404 |
Asset base path, missing build output, wrong served directory, or stale filename reference | Compare the requested URL with the build output and deployed files; verify the host’s output directory and deployment prefix. |
| In-app navigation works, but direct navigation or refresh returns 404 | Server or platform does not route the client-side path to the app | Configure a host-appropriate fallback to index.html for app routes, while preserving asset and API handling. |
| The failure starts after a release, often for a hashed chunk | HTML and bundle files may not align, or a browser or separate CDN may retain cached content | Compare the HTML-referenced hash with the active deployment’s files, then inspect relevant cache layers. |
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