JS MIME Type

About JavaScript files

RFC 9239 (2022) made text/javascript the official type and marked application/javascript and application/x-javascript as obsolete; browsers accept all of them. Module scripts (<script type="module"> and import) are strict: if the server answers with any non-JavaScript type, the import fails.

• “Failed to load module script: Expected a JavaScript module script but the server responded with a MIME type of text/html” means the request got an HTML page, typically a 404 or an SPA rewrite that sends index.html for every path.

Other types seen for these files: application/javascript , application/x-javascript . Accept them when reading uploads, but send text/javascript yourself.

File signature

JS has no fixed signature: it is text without a magic number, so validate uploads by parsing the content rather than by checking bytes.

Serve .js with the right Content-Type

Content-Type: text/javascript; charset=utf-8

text/javascript js cjs;

AddType text/javascript .js .cjs
AddCharset UTF-8 .js .cjs

@js path *.js *.cjs
header @js Content-Type "text/javascript; charset=utf-8"

curl -sI https://example.com/file.js | grep -i '^content-type'
# Content-Type: text/javascript; charset=utf-8

Set the type in code

res.set('Content-Type', 'text/javascript; charset=utf-8');

import mimetypes

mimetypes.add_type('text/javascript', '.js')  # if your system doesn't know it
mimetypes.guess_type('file.js')[0]  # 'text/javascript'

mime.AddExtensionType(".js", "text/javascript; charset=utf-8")
w.Header().Set("Content-Type", "text/javascript; charset=utf-8")

Accept .js uploads

<input type="file" accept=".js,.cjs,text/javascript">

The accept attribute only filters the file picker; users can still choose another file. Listing the extension as well as the type matters, because operating systems don’t always report the type you expect.

Related types

• .mjs (text/javascript): JavaScript module
• .css (text/css): CSS stylesheet
• .wasm (application/wasm): WebAssembly module
• .ts (video/mp2t): MPEG transport stream

More web

.html , .svg , .webmanifest , .rss , .vtt . Look up any extension or type with the MIME type lookup.

Sources

• IANA registration: text/javascript
• MDN: Common media types

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Frequently asked questions