There are several ways you can do this, but one way might be best for certain situations (e.g. within an iFrame).
Protocol + Domain + Page
document.URL
> "http://example.com/page1.html"
document.location.href
> "http://example.com/page1.html"
Protocol + Domain
document.location.origin
> "http://example.com"
Domain
document.location.host
> "example.com"
Page
document.location.pathname
> "/page1.html"