javascripthtmliframecross-domain

What is window.origin?

What is window.origin? It doesn't seem to be documented in the usual place.

It looks like it might be very similar to window.location.origin - for example, here on Stack Overflow, both return

https://stackoverflow.com

But inside an iframe, they're different:

console.log(window.location.origin);
console.log(window.origin);

https://stacksnippets.net
null

The embedded snippet is inside an iframe without allow-same-origin. If you change the iframe, for example, if you edit Stack Overflow's HTML and manually add the attribute:

<iframe name="313b857b-943a-7ffd-4663-3d9060cf4cb6" sandbox="allow-same-origin allow-forms allow-modals allow-scripts" class="snippet-box-edit" frameborder="0" style="">
                                                             ^^^^^^^^^^^^^^^^^^

and then run the snippet, you get:

https://stacksnippets.net
https://stacksnippets.net

The same sort of behavior is exhibited on other sites with <iframe>s.

Google does not appear to have any authoritative links on the subject. Searching for the exact phrase + Javascript gives many results related to iframes and postMessage, but no precise description of what window.origin actually is.

Calling postMessage from a child iframe appears to result in the parent window receiving a message with the origin property matching the window.origin of the child frame - without allow-same-origin, it's null, otherwise it looks like it's the same as the window.location.origin of the child.

The above is what I think I've figured out from guessing-and-checking, but I'm nowhere near certain. I'd appreciate a confirmation/explanation, preferably with a link to an authoritative source.

Solution

  • WindowOrWorkerGlobal.origin returns the origin of the environment, Location.origin returns the origin of the URL of the environment.

    Unfortunately Stack-Snippets null-origined frames will make for a confusing example...

    At the risk of paraphrasing the specs themselves, let's say we are on https://example.com and from there, we create a new <iframe> element without an src attribute:

    var frame = document.createElement("iframe")
frame.onload = function() {
  var frameWin = frame.contentWindow;
  console.log(frameWin.location.href); // "about:blank"
  console.log(frameWin.location.origin) // "null"
  console.log(frameWin.origin) // "https://example.com"
}
document.body.appendChild(frame);

    Live example

    The location of our frameWin is "about:blank" and its location.origin is "null", because "about:blank" is an opaque origin.

    However, the frame's Window frameWin got its own origin set to the one of the parent Window ("https://example.com") which was set when frameWin's browsing context got initialized.

    If you wish a little diving into the specs here are the relevant steps for the previous example:

    If the element has no src attribute specified, or its value is the empty string, let url be the URL "about:blank".

    If invocationOrigin is not null, and url is about:blank, then return invocationOrigin.

    So here it has been determined that origin of the new browsing context is invocationOrigin, i.e the origin of the browsing context that did create frame, while url, used by location, is "about:blank".

    Now the case of StackSnippets sandboxed frames is a bit particular in that they do have an src and thus a tuple-origin url, but since their sandbox attribute makes their origin opaque, they'll behave at the inverse of what is exposed in the previous example, making self.origin return "null" and location return the iframe's src's URL.