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
  )
}

This is what I have come up with:

function isXDomain(dURL, sURL) {
  var HOSTNAME = /^(?:https?:)?(?:\/\/)?([^\/]+)\//;
  sURL || (sURL = location.href.match(HOSTNAME));
  dURL && (dURL = (dURL + '').match(HOSTNAME));
  return sURL && !!dURL && sURL[1] === dURL[1];
}

the second parameter is optional, if one wants to compare against a secondary URL instead of the current "location.href".

The above code also accepts the following as valid URL:

bar.com/pages/page.html

not much shorter though.

..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.