Caching is frequently used for static resources and taken care of by a file-serving module or web server. However, there's nothing stopping us from implementing caching with dynamic resources as well.

The ETag is a cache mechanism for HTTP requests, which allows the client to make conditional requests.

There are several NPM modules that handle ETag generation and conditional requests, but we can implement our own Express middleware to do this in a few lines of code. We need to set the ETag and check whether the current request matches that ETag. If it does, we will send 304 Not Modified, and if it doesn't, we will send the whole body. The specification says that we shouldn't include content headers when sending 304 Not Modified, so we will remove them if they exist. The full code for this middleware...