Why response 304 doesn't have Content-Type header?

Question

I've been playing with express.js trying to return simple json object and noticed that even though I explicitly set Content-Type header to be application/json it is only visible on first response when status code is 200. Every following response with 304 won't have Content-Type header.

My code sample:

app.get('/user', function (req, res) {
    res.set('Content-Type', 'application/json');

    res.send([
        { user: "john", email: "[email protected]"},
        { user: "marry", email: "[email protected]"},
        { user: "dan", email: "[email protected]"}
    ]);
});

What is the reason for that?

Answer 1

304 Not Modified means that the request contained a conditional header asking the server to respond with the contents of the resource only if the the resource has been modified.

Since no content is being returned, the Content-Type header is not sent. This is the recommended behavior for a 304 Not Modified HTTP reply.

From RFC 7232 §4.1 :

The server generating a 304 response MUST generate any of the
following header fields that would have been sent in a 200 (OK)
response to the same request: Cache-Control, Content-Location, Date,
ETag, Expires, and Vary.

Since the goal of a 304 response is to minimize information transfer when the recipient already has one or more cached representations, a sender SHOULD NOT generate representation metadata other than the above listed fields unless said metadata exists for the purpose of
guiding cache updates (e.g., Last-Modified might be useful if the
response does not have an ETag field).

I don't know anything about express.js, but it I would look into what sort of caching is being done.