Q:

Get domain name without subdomains using JavaScript?

How to get the domain name without subdomains?
e.g. if the url is "http://one.two.roothost.co.uk/page.html" how to get "roothost.co.uk"?

A:

Following is a solution to extract a domain name without any subdomains. This solution doesn't make any assumptions about the URL format, so it should work for any URL. Since some domain names have one suffix (.com), and some have two or more (.co.uk), to get an accurate result in all cases, we need to parse the hostname using the Public Suffix List, which contains a list of all public domain name suffixes.

Solution
First, include the public suffix list js api in a script tag in your HTML, then in JavaScript to get the hostname you can call:
var parsed = psl.parse('one.two.roothost.co.uk');
console.log(parsed.domain);

...which will return "roothost.co.uk". To get the name from the current page, you can use location.hostname instead of a static string:
var parsed = psl.parse(location.hostname);
console.log(parsed.domain);

Finally, if you need to parse a domain name directly out of a full URL string, you can use the following:
var url = "http://one.two.roothost.co.uk/page.html";
url = url.split("/")[2]; // Get the hostname
var parsed = psl.parse(url); // Parse the domain
document.getElementById("output").textContent = parsed.domain;

JSFiddle Example (it includes the entire minified library in the jsFiddle, so scroll down!): https://jsfiddle.net/6aqdbL71/2/

