Can I use fetch to access a localhost server?

Yes!

Take this server for example:

Bun.serve({
  fetch() {
    return new Response('Bun!');
  },
});

Run using bun server.js and hit http://localhost:3000 in your browser to make sure the server works.

Next up, open any page, like Google, open the developer tools, go to the Console tab and run this:

fetch('http://localhost:3000').then(response => response.text()).then(text => console.log(text));

Your fetch call is still subject to CORS so you'll get this error:

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at
http://localhost:3000/. (Reason: CORS header ‘Access-Control-Allow-Origin’ missing).

To fix this, make sure your local server serves the appropriate ACAO header:

Bun.serve({
  fetch(req) {
    const headers = { 'Access-Control-Allow-Origin': 'https://www.google.com' };
    return new Response("Bun!", { headers });
  },
});

With this, your Google tab can now make fetch calls to your localhost server. Other websites won't be able to as they won't pass the CORS check.

Change this to your web app's orgin and now your live web app can hit the users' local server and prompt them to install and start one if the web app expects that to work and the user hasn't done it.

To allow any and all websites to call your localhost server, use this:

Bun.serve({
  fetch(req) {
    const headers = { 'Access-Control-Allow-Origin': '*' };
    return new Response("Bun!", { headers });
  },
});

This is not a good idea.