What is window.origin?

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:

• At frame creation:
  https://html.spec.whatwg.org/multipage/iframe-embed-object.html#the-iframe-element:about:blank

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

• When creating a new browsing context for frame
  https://html.spec.whatwg.org/multipage/browsers.html#creating-browsing-contexts: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.