Restrictions of XMLHttpRequest's getResponseHeader()?

Question

I've noticed that the results of and XMLHttpRequest.getResponseHeader() don't always match the real headers returned (if the request is made in a regular manner).

For example, assume I'm making an xhr request for https://foo.example.com/api/resource/100. In Chrome's developer console, under 'Network', I can see the response being made -- I can also see all of the response headers (say, 10). However (copy-pasted console):

> response   XMLHttpRequest > response.getAllResponseHeaders();   "content-type: text/html   "  

Are there any restrictions on what headers are available? Is this dependent on the response type? I remember getting a complete set of headers for 404s but just this one for 400s.

What gives?

Answer 1

The current state of standardizing the XMLHttpRequest API does only restrict the access to the Set-Cookie and Set-Cookie2 header fields:

client.getAllResponseHeaders()

Returns all headers from the response, with the exception of those whose field name is Set-Cookie or Set-Cookie2.

Any other header field should be returned.

But as you’re doing a cross-origin request, the browser needs to implement XMLHttpRequest Level 2 as the original XMLHttpRequest does only allow same-origin requests:

The XMLHttpRequest Level 2 specification enhances the XMLHttpRequest object with new features, such as cross-origin requests […]

There you can read that the “Cross-Origin Resource Sharing specification filters the headers that filters the headers that are exposed by getResponseHeader() for non same-origin requests.”. And that specification forbids access to any response header field other except the simple response header fields (i.e. Cache-Control, Content-Language, Content-Type, Expires, Last-Modified, and Pragma):

User agents must filter out all response headers other than those that are a simple response header […]

E.g. the getResponseHeader() method of XMLHttpRequest will therefore not expose any header not indicated above.