From the XMLHttpRequest spec:
For 304 Not Modified responses that are a result of a user agent
generated conditional request the user agent must act as if the server
gave a 200 OK response with the appropriate content.
In other words, the browser will always give status code 200 OK, even for requests that hit the browser cache.
However, the spec also says:
The user agent must allow author request headers to override automatic cache
validation (e.g. If-None-Match or If-Modified-Since), in which case
304 Not Modified responses must be passed through.
So, there is a workaround to make the 304 Not Modified responses visible to your JavaScript code.