setting cross-subdomain cookie with javascript

In my case we needed to set a cookie that would work across our .com subdomains:

function setCrossSubdomainCookie(name, value, days) {
  const assign = name + "=" + escape(value) + ";";
  const d = new Date();
  d.setTime(d.getTime() + (days*24*60*60*1000));
  const expires = "expires="+ d.toUTCString() + ";";
  const path = "path=/;";
  const domain = "domain=" + (document.domain.match(/[^\.]*\.[^.]*$/)[0]) + ";";
  document.cookie = assign + expires + path + domain;
}

This might not work for .co.uk etc but the principle can be used

Here is a link on how to share cookies amongst a domain:

https://www.thoughtco.com/javascript-by-example-2037272

It involves setting the domain attribute of the cookie string like:

document.cookie = "myValue=5;path=/;domain=example.com";

This cookie should now be accessible to all sub domains of example.com like login.example.com