response-is

The Content-Type header can sometimes be unclear. This library aims to help you determine the type of content you're dealing with.

Disclaimers

• This module is exclusively an ESM module.
• This module is intended for server side use. It will not work in the browser: The browser "fetch" does not have access to the response headers.

Example

import { responseIs } from "response-is";
const response = await fetch("https://omrilotan.com");
const is = responseIs(response);

if (is.html) {
  // do something
} else if (is.json) {
  // do something else
} else {
  // do something else
}

Functionality

Available getters

html, xhtml, form, json, xml, text, stylesheet, javascript, font, image, icon, video, audio, stream, binary, bmp, css, gif, ico, jpeg, jpg, js, mjs, mid, midi, mp3, mp4, mpeg, ogg, otf, png, svg, svgz, ttf, webm, webp, woff, woff2

Getter truthfulness is not nessecarily mutually exclusive. For example, the Content-Type "application/xhtml+xml" will return true for both is.html and is.xhtml, while the Content-Type "text/html" will return true for is.html but not for is.text. The Content-Type "text/javascript" will return true for is.javascript, is.js and is.text, so make sure you prioritize your checks. For example is.javascript should be checked before is.text.

Additional functions

• toJSON - returns a JSON representation of the content type
• toString - returns the value of the content type header

const is = responseIs(
  new Response(null, headers: new Headers({
    "content-type": "text/html"
  }))
);
is.toJSON(); // { mediaType: "text", subType: "html" }

toString - returns the value of the content type header

const is = responseIs({
  headers: {
    "content-type": "text/html; charset=utf-8",
  },
});
is.toString(); // text/html

Use Case

This example use case detects "Cache Deception" attack where users create a URL with the suffix ".js" while the route returns actual data.

import { responseIs } from "response-is";

const CACHABLE_EXTENSIONS = ["js", "css", "mjs"]; // See default cache behaviour: https://developers.cloudflare.com/cache/concepts/default-cache-behavior/

export default {
  async fetch(request: Request, env: Env, ctx: ExecutionContext): Promise<Response> {
    const response = await fetch(request);
    const url = new URL(request.url);
    const extension = url.pathname.split(".").pop();

    if (!CACHABLE_EXTENSIONS.includes(extension)) {
      const is = responseIs(response);
      if (!is[extension] === false) {
        // clear from cache
        const cache = caches.default;
        ctx.waitUntil(cache.delete(request.url));
        response.headers.set("Cf-Cache-Status", "BYPASS");
      }
    }
    return response;
  },
};