To find why a JavaScript file or chunk fails after deployment, compare the URL the browser requests with the URL your build is meant to emit, the file’s actual location in the deployed output, and the server’s response. A 404 for a client-side page route is a different problem from a missing JavaScript asset: the former may need a hosting rewrite; the latter needs the requested file and path checked.
Trace the failing request in the deployed app
Use the actual production or preview URL, including the route and any subpath where the app is mounted. A development server can behave differently because production builds may rewrite asset URLs or add hashed filenames. For example, Vite documents an imported asset URL changing from a source path in development to a hashed output path in production (Vite: Static Asset Handling).
- Open Chrome DevTools → Network, turn on recording, and reload the page. Filter for JS, then select the failed request.
- Record the complete Request URL and status. Check the request type and initiator, then inspect Headers and Response. The initiator can help show whether a script came from the document parser or was requested by JavaScript.
- Interpret the result before changing configuration. A 404 means the requested URL did not return the expected resource; it does not by itself establish whether the file is missing, the prefix is wrong, or the server/CDN mapping is wrong. CORS or blocked-origin errors should be investigated through the browser’s status and response headers, not treated automatically as a bad path.
- Repeat with cache disabled. In DevTools, select Disable cache while DevTools is open, then reload. Chrome also provides an empty-cache hard reload. Compare the document and asset requests with the uncached result to check whether an older cached page points to old filenames.
Chrome’s Network panel documents request details, initiators, response headers and bodies, blocked requests, and cache controls (Chrome DevTools: Network features reference).
Work out how the browser resolves the URL
Script URLs in HTML
For a script URL written in HTML, start with the document’s URL and any applicable base URL. A relative URL is not inherently tied to the source file’s directory; its resolution depends on the document context. Compare the literal value in the HTML with the complete URL shown in Network.
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ES module imports
For a relative JavaScript module specifier, resolution uses the document base URL, and an import map can remap module specifiers. The text of an import alone therefore does not prove which network URL the browser will request. Check the resolved request in Network and compare it with the import map and page’s base context. See MDN: JavaScript modules.
Generated entry files and lazy-loaded chunks
If the entry script loads but a later chunk fails, inspect that chunk’s request and initiator separately. The entry URL and the runtime’s chunk URL can be based on different generated references or a runtime public path. A successful entry request does not establish that every lazily loaded file uses the correct prefix.
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Check the build tool’s public path
Look for the setting that controls production asset URLs in the build tool actually used by the project. Similar concepts have different names and behaviors; do not assume configuration from one tool applies to another.
| Tool | Setting or behavior to check | Important detail |
|---|---|---|
| Vite | base in the build configuration |
Set the public base path for a nested deployment. Vite rewrites asset references during build. For runtime URL construction, its documented spelling is import.meta.env.BASE_URL; that value is statically replaced and must use this exact property form. Relative bases such as ./ or an empty string make generated URLs relative to each file and require import.meta support. See Vite: Building for Production. |
| webpack | output.publicPath |
Sets the URL prefix used for emitted assets. If you set the public path at runtime, do so before application code that needs to load dependent assets. See webpack: Asset Modules, public path. |
| Vue CLI | publicPath |
For deployment away from the domain root, Vue CLI documents prefixing public assets with this setting. Its guidance uses BASE_URL in HTML templates and process.env.BASE_URL in app code. This is Vue CLI-specific. See Vue CLI: HTML and Static Assets. |
For a Vite app deployed under a subpath, compare the configured base with the real mount path. A root-absolute URL such as /icon.png starts at the domain root, not automatically at the app’s subpath.
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Confirm the requested file exists where the URL points
Inspect the build output and deployed files, not just the source tree or configuration. Confirm the exact requested filename exists and that deployment or CDN mapping preserves the expected directory prefix. A correct base-path setting cannot make a file available if it was omitted from the output or deployment.
Vite distinguishes imported assets from files in its public directory. Imported assets resolve to public URLs and can be transformed for production. Files in public are copied to the output root and are referenced with root-absolute paths such as /icon.png (Vite: Static Asset Handling). If the app is served from a subpath, check whether those root-absolute references match the intended deployment layout.
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Tell an asset 404 from a client-route 404
Check which URL failed. If the document loads but direct navigation to a path such as /some/client/route returns a server 404, the host may need an SPA fallback or rewrite that serves the application entry document for client-side routes. That is not the same as a missing .js file: for a failed asset request, verify the asset URL, built file, and host or CDN mapping. Vercel describes its routing as server-resolved unless SPA routing is configured; the specific rewrite depends on the host (Vercel: Why is my deployed project giving 404?).
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Use the symptom to choose the next check
| What you see | What to check next |
|---|---|
| Most or all asset requests have the same wrong prefix | Compare the configured build base/public path with the app’s actual deployment mount path. |
| The entry script loads but a dynamic chunk fails | Inspect the chunk’s full URL and initiator; check generated references and runtime public-path configuration. |
| A JavaScript-looking URL returns 404 | Compare the full URL with the deployed file layout and server/CDN mapping; the status alone cannot identify which is wrong. |
| The app loads at its home page, but direct navigation to a client route returns 404 | Check the host’s SPA rewrite or fallback behavior. |
| The browser reports CORS or a blocked request | Inspect the browser status and response headers to identify the origin or policy failure before altering the URL. |
| Only some visits use old filenames or paths | Compare cache-disabled requests with normal requests and check whether cached HTML references an earlier build. |
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