What is the MIME type of .mjs files?
The MIME type (media type) for JavaScript module files with the .mjs extension is text/javascript. You may also see application/javascript. The same type is used for .js. The .mjs extension marks ES modules in Node.js. Some older servers do not know it and send application/octet-stream, which browsers reject for module scripts — add the type explicitly.
Content-Type header
Content-Type: text/javascript; charset=utf-8Text-based types should include a charset so the browser decodes characters correctly.
Server configuration
| Server | Configuration |
|---|---|
Nginx (mime.types or types {}) | text/javascript mjs; |
Apache (.htaccess) | AddType text/javascript .mjs |
IIS (web.config) | <mimeMap fileExtension=".mjs" mimeType="text/javascript" /> |
| Node.js / Express | res.type('mjs') or res.setHeader('Content-Type', 'text/javascript; charset=utf-8') |
| Amazon S3 | Set Content-Type: text/javascript on the object (metadata), or pass --content-type text/javascript to aws s3 cp |
Should .mjs files be compressed?
Yes. JavaScript module is text, so enabling gzip or Brotli for text/javascript typically saves 50–80 % of the transfer size. Make sure your server’s compression list includes this type — check with the website compression test.
Why the MIME type matters
Browsers decide what to do with a response — render, run, play or download — from the Content-Type header, not the file extension. With X-Content-Type-Options: nosniff, a wrong type causes stylesheets and scripts to be blocked outright. For uploads, never trust the type sent by the client; check the file’s content (its magic bytes) on the server.
Frequently asked questions
What is the MIME type for .mjs?
text/javascript. Alternatives seen in the wild: application/javascript.How do I set the Content-Type for .mjs in Nginx?
text/javascript mjs; is listed in the types block (usually in /etc/nginx/mime.types) and reload Nginx.