Introduction
Caching plays a crucial role in optimizing web performance. When Output Caching is enabled in IIS for specific file extensions or URLs, it deliver 304 Not Modified status code. This status code is essential for managing caching, as it indicates whether a resource has been modified since the last time it was requested.
While browsing the site it is evident that requests for .js files return a 304 Not Modified status code, along with that the server sending an ETag which was matched with the Request Header If-None-Match.
Since the purpose of a 304 response is to minimize data transfer when the client already holds a cached version, the server avoids including metadata other than the required fields unless that metadata is necessary for cache management.
Conclusion
The HTTP 304 Not Modified status code is used for web caching and performance optimization. It notifies client to use an existing cached copy considering it is able to validate the specified condition catch condition in request header.
References
HTTP status code overview - Internet Information Services | Microsoft Learn
RFC 9110 - HTTP Semantics (ietf.org)