We have a string with an URL. And we want to get the host URL. I don't know about the built-in functions for this and I don't think that you should find external libraries for this. Although, it's a good idea to create your own personal class.

But now I will write down only how to extract the host URL out of the string with full URL.

const _absoluteUrl = "http://mySuperSite.com/sites/Mysites/Files/default.aspx"

First of all, let's split the string with '/' delimiter to have an array of the parts

const hostParts = _absoluteUrl.split('/')

The result is this one

{ hostParts:
   [ 'http:',
     '',
     'mySuperSite.com',
     'sites',
     'Mysites',
     'Files',
     'default.aspx' ] }

As you can see, now we need just to join the 1st and the 3rd elements of the array.

const host = hostParts[0] + '//' + hostParts[2]

That's all folks 🙂 The result is: 'http://mySuperSite.com'

And now let's wrap it in a function

const GetHostFromUrl = (fullUrl) => {
    const hostParts = fullUrl.split('/')
    if(hostParts.length > 1) return hostParts[0] + '//' + hostParts[2]
    return `The string doesn't contain a full URL`
    
}
const _absoluteUrl = "http://mySuperSite.com/sites/Mysites/Files/default.aspx"
const myhost = GetHostFromUrl(_absoluteUrl)