get-apex-domain

Returns the apex domain (aka base, bare, naked, root apex, or zone apex domain) of the current web page without the use of a public suffix list. The apex domain is also the top-most domain that allows for setting cookies.

Because of the methodology used, this library is very small.

What's an apex domain?

An apex domain is the longest portion of a domain that does not contain a subdomain. In the case of store.example.co.uk, example.co.uk would be the apex domain.

When a website owner attempts to register a domain name, they cannot register a domain matching an effective top-level domain (e.g., com, co.uk) due to restrictions controlled by registrars. However, domains ending in an effective top-level domain are allowed (e.g., example.com, example.co.uk). This is the apex domain. The domain name owner can then create subdomains ending in the domain they registered (e.g., store.example.com, store.example.co.uk), but subdomains are not apex domains.

Requirements

This will only work in a browser and cookies must be enabled.

How detection is accomplished

As it turns out, browsers allow websites to set cookies for apex domains (e.g., example.co.uk) but not top-level domains (e.g., uk) or effective top-level domains (e.g., co.uk).

Considering a hostname of store.example.co.uk, we can determine the apex domain by attempting to write a cookie on uk, then co.uk, then example.co.uk, then store.example.co.uk, until we succeed in writing a cookie. In this case, the first attempt that will succeed is example.co.uk. This is the apex domain.

Installation

If you use npm for package management, you can install get-etld by running the following command from within your project's directory:

npm install get-apex-domain

Usage

import getApexDomain from "get-apex-domain";

console.log(getApexDomain());

If cookies are not enabled, an error will be thrown.

The localhost hostname presents a unique case. If the hostname is localhost, the returned apex domain will be undefined.

Special Recognition

Full credit goes to Joe Khoury for devising the strategy used to determine the apex domain.