Get subdomain and load it to url with greasemonkey

Question

var full = window.location.host
//window.location.host is subdomain.domain.com
var parts = full.split('.')
var sub = parts[0]
var domain = parts[1]
var type = parts[2]
//sub is 'subdomain', 'domain', type is 'com'
var newUrl = 'http://' + domain + '.' + type + '/your/other/path/' + subDomain
window.open(newUrl);

The answer provided by Derek will work in the most common cases, but will not work for "xxx.xxx" sub domains, or "host.co.uk". (also, using window.location.host, will also retrieve the port number, which is not treated : http://www.w3schools.com/jsref/prop_loc_host.asp)

To be honest I do not see a perfect solution for this problem. Personally, I've created a method for host name splitting which I use very often because it covers a larger number of host names.

This method splits the hostname into {domain: "", type: "", subdomain: ""}

function splitHostname() {
        var result = {};
        var regexParse = new RegExp('([a-z\-0-9]{2,63})\.([a-z\.]{2,5})$');
        var urlParts = regexParse.exec(window.location.hostname);
        result.domain = urlParts[1];
        result.type = urlParts[2];
        result.subdomain = window.location.hostname.replace(result.domain + '.' + result.type, '').slice(0, -1);;
        return result;
}

console.log(splitHostname());

This method only returns the subdomain as a string:

function getSubdomain(hostname) {
        var regexParse = new RegExp('[a-z\-0-9]{2,63}\.[a-z\.]{2,5}$');
        var urlParts = regexParse.exec(hostname);
        return hostname.replace(urlParts[0],'').slice(0, -1);
}

console.log(getSubdomain(window.location.hostname));
// for use in node with express:  getSubdomain(req.hostname)

These two methods will work for most common domains (including co.uk) NOTE: the slice at the end of sub domains is to remove the extra dot.

I hope this solves your problem.

The solutions provided here work some of the time, or even most of the time, but not everywhere. To the best of my knowledge, the best way to find the full subdomain of any domain (and remember, sometimes subdomains have periods in them too! You can have sub-subdomains, etc) is to use the Public Suffix List, which is maintained by Mozilla.

The part of the URL that isn't in the Public Suffix List is the subdomain plus the domain itself, joined by a dot. Once you remove the public suffix, you can remove the domain and have just the subdomain left by removing the last segment between the dots.

Let's look at a complicated example. Say you're testing sub.sub.example.pvt.k12.ma.us. pvt.k12.ma.us is a public suffix, believe it or not! So if you used the Public Suffix List, you'd be able to quickly turn that into sub.sub.example by removing the known suffix. Then you could go from sub.sub.example to just sub.sub after stripping off the last portion of the remaining pieces, which was the domain. sub.sub is your subdomain.