hostnameRoot.js -- returns URL/location.hostname with certain prefixes removed: m. mob. mobi. mobile. www. www[0-9]. ww[0-9].

hostnameRoot.js

// hostnameRoot.js -- returns URL/location.hostname with certain prefixes removed: m. mob. mobi. mobile. www. www[0-9]. ww[0-9].

const hostnameRoot = hostname => String(hostname ?? "")
.toLowerCase()
.replace(
  /^(m(ob(i(le)?)?)?|ww(w?\d|w))\.(.+?\..+)/,
  "$6"
);

console.log([
  hostnameRoot("m.youtube.com") === "youtube.com",
  hostnameRoot("moB.youtube.com") === "youtube.com",
  hostnameRoot("mobI.youtube.com") === "youtube.com",
  hostnameRoot("mobilE.youtube.com") === "youtube.com",
  hostnameRoot("wwW.youtube.com") === "youtube.com",
  hostnameRoot("www1.youtube.com") === "youtube.com",
  hostnameRoot("ww2.youtube.com") === "youtube.com",
  
  hostnameRoot("ww.youtube.com") === "ww.youtube.com",
  hostnameRoot("youtubE.com") === "youtube.com",
  hostnameRoot("immobile.youtube.com") === "immobile.youtube.com",
  hostnameRoot("mo.youtube.com") === "mo.youtube.com",
  hostnameRoot("mobil.youtube.com") === "mobil.youtube.com",

  hostnameRoot("m.com") === "m.com",
  hostnameRoot("mobile.com") === "mobile.com",
  hostnameRoot("www.com") === "www.com",
  
  hostnameRoot() === "",
  hostnameRoot(null) === "",
  hostnameRoot(0) === "0",
  hostnameRoot([]) === "",
  hostnameRoot({}) === "[object object]",
]);