Identify the specific module request
Inspect the failed module response: a missing asset rewritten to HTML needs a different fix from a real script served with the wrong type.
A missing build asset can cause the same browser MIME complaint as a type configuration error. In the Network view, record the failed module URL, HTTP status, Content-Type and any redirect, without copying private page data. MDN explains that browser modules require a JavaScript-compatible MIME type, such as text/javascript. A text/html response is usually worth inspecting before any server-type change.
Confirm the intended static build and exact host. A wrong base path, filename case or missing emitted file can send an asset request to an application fallback page. A module that genuinely exists but is served with the wrong type is a different branch.
Distinguish missing file from wrong type
Compare the module URL in the served HTML with the deployable build’s asset manifest or output tree. If the file does not exist at that path, correct the application’s asset base, deployment folder or build upload; do not merely label the entry HTML as JavaScript. If the response is a 404 or a 200 containing the application entry page, review any SPA fallback rule. A fallback meant for page routes must not mask missing asset paths; the related deep-link guide requires a deliberate absent-asset 404 test.
If the module file does exist and its response body is the expected JavaScript but Content-Type is wrong, ask whether the purchased account exposes a scoped MIME mapping or whether the contracted platform operator must correct that extension. Do not alter global web-server mappings on shared hosting. Check .mjs separately if the build uses it; support for an extension is not inferred from support for .js.
Change the responsible layer only
The application owner should re-upload the missing build file or fix its generated URL, then verify the file path and size. For a genuine MIME configuration fault, provide the operator the exact URL, extension, current response header and expected file type so they can review the account route. Keep the previous build and any account rule for rollback. Avoid forcing JavaScript MIME on all unknown paths, which could mislabel HTML or downloads.
Reload and test the actual interaction
Open the module URL directly and confirm it returns the intended file with a compatible JavaScript Content-Type and a successful status. Reload the page in a fresh browser session and exercise the previously broken interaction. Also test one existing stylesheet and an intentionally missing asset. If another asset now fails or missing paths are swallowed by HTML, roll back the build or account rule and narrow the fix.
For example, a module request returning 200 and an HTML entry page is still unusable as JavaScript. Check whether the asset exists at the built path before changing MIME rules or the app fallback.
Frequently asked questions
Why does a missing module return HTML? A front-end route fallback or wrong deployment path can send the application entry page for an asset URL.
Can I change only the MIME header? Only when the requested file is truly the intended JavaScript. A header cannot turn an HTML error page into a working module.
Sources and technical references
Related guides
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