To tell what the web server is, right inside of a webpage

Sometimes if we start a local webserver, or if we use other online pages, we would like to find out what webserver it is, and it is possible to do it right inside of the webpage. We can fetch something, and look at the response header by res.headers.get("Server"):

fetch("foo.txt").then((res) => {
  document.querySelector(
    "#server-note"
  ).innerHTML = `The server is ${JSON.stringify(res.headers.get("Server"))}`;
});

Demo: https://jsfiddle.net/KennethKinLum/80td3s4f/

The server for

Webserver	in the header
Node.js's live-server	nothing (null)
Ruby's ruby -run -e httpd . -p 8080	WEBrick/1.4.2 (Ruby/2.6.3/2019-04-16)
Python 2's python -m SimpleHTTPServer 8080	SimpleHTTP/0.6 Python/2.7.16
Python 3's python3 -m http.server 8080	SimpleHTTP/0.6 Python/3.8.2
JSFiddle	nginx
Codesandbox	cloudflare
macOS Big Sur's webserver	Apache/2.4.46 (Unix)

It is possible to tell the user-agent as well, using navigator.userAgent.