Get the MIME type of a file

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Serving a file over HTTP needs a content type header. The standard library maps file extensions to MIME types. This example looks up the type for a few file names.

import { contentType } from "jsr:@std/media-types";
import { extname } from "node:path";

Look up the MIME type for a file name by its extension.

console.log(contentType(extname("photo.png"))); // image/png
console.log(contentType(extname("data.json"))); // application/json

Text types include their charset, ready to use in a header.

console.log(contentType(extname("index.html"))); // text/html; charset=UTF-8

Unknown extensions return undefined, so provide a fallback. The standard fallback for arbitrary binary data is application/octet-stream.

const type = contentType(extname("archive.xyz")) ??
  "application/octet-stream";
console.log(type); // application/octet-stream

A typical use: serving content with the right header.

const path = "data.json";
const body = new Blob(['{ "hello": "world" }']).stream();
const response = new Response(body, {
  headers: {
    "content-type": contentType(extname(path)) ?? "application/octet-stream",
  },
});
console.log(response.headers.get("content-type")); // application/json

Note that the lookup trusts the file name. To identify a file by its actual contents, inspect its magic bytes instead.

Run this example locally using the Deno CLI:

deno run https://docs.deno.com/examples/scripts/mime_type.ts

Get the MIME type of a file

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