Get the domain name of the subdomain Javascript

var parts = location.hostname.split('.');
var subdomain = parts.shift();
var upperleveldomain = parts.join('.');

To get only the second-level-domain, you might use

var parts = location.hostname.split('.');
var sndleveldomain = parts.slice(-2).join('.');

The accepted answer will work to get the second level domain. However, there is something called "public suffixes" that you may want to take into account. Otherwise, you may get unexpected and erroneous results.

For example, take the domain www.amazon.co.uk. If you just try getting the second level domain, you'll end up with co.uk, which is probably not what you want. That's because co.uk is a "public suffix", which means it's essentially a top level domain. Here's the definition of a public suffix, taken from https://publicsuffix.org:

A "public suffix" is one under which Internet users can (or historically could) directly register names.

If this is a crucial part of your application, I would look into something like psl (https://github.com/lupomontero/psl) for domain parsing. It works in nodejs and the browser, and it's tested on Mozilla's maintained public suffix list.

Here's a quick example from their README:

var psl = require('psl');

// TLD with some 2-level rules.
psl.get('uk.com'); // null);
psl.get('example.uk.com'); // 'example.uk.com');
psl.get('b.example.uk.com'); // 'example.uk.com');

This is faster

const firstDotIndex = subDomain.indexOf('.');
const domain = subDomain.substring(firstDotIndex + 1);