Crossdomain url test

isXDomain.js

/*

  document.location of the current page tested against a url (string) like:

  "pages/page.html" // false 
  "../pages/page.html" // false
  "/pages/page.html" // false
  "//foo.bar.com/pages/page.html" // false
  "http://foo.bar.com/pages/page.html" // false
  "//bar.com/pages/page.html" // true
  "https://bar.com/pages/page.html" // true

*/

function isXDomain (url) {
  return !(
    url.match(/^((\.\.\/)|([^:/]+\/)|(\/[^/]))/) ||
    (url.match(/^\/\//) || url.indexOf(location.protocol) == 0) &&
    url.indexOf("//" + location.hostname) > -1
  )
}

..but shorter nevertheless :D

The added secondary URL is nice (especially considering the shortness).

But should "bar.com/pages/page.html" be considered a valid URL considering a <a href="bar.com/pages/page.html">My page</a> on "foo.com/index.html" would point to "foo.com/bar.com/pages/page.html"?? Depends on the strictness I guess...

Haven't checked about the validity of that URL, however try changing the RE to:

var HOSTNAME = /\/\/([^\/]+)\//;

to avoid that dubious condition and see if it fits exactly all your tests/needs.