If you wish a full domain origin, you can use this:
document.location.origin
And if you wish to get only the domain, use can you just this:
document.location.hostname
But you have other options, take a look at the properties in:
document.location
Reference:
https://stackoverflow.com/questions/11401897/get-the-current-domain-name-with-javascript-not-the-path-etc/11401921