Building Enterprise JavaScript Applications

Moving common logic into middleware

Let's see how we can improve our code further. If you examine our Create User endpoint handler, you may notice that its logic could be applied to all requests. For example, if a request comes in carrying a payload, we expect the value of its Content-Type header to include the string application/json, regardless of which endpoint it is hitting. Therefore, we should pull that piece of logic out into middleware functions to maximize reusability. Specifically, these middleware should perform the following checks:

• If a request uses the method POST, PUT or PATCH, it must carry a non-empty payload.
• If a request contains a non-empty payload, it should have its Content-Type header set. If it doesn't, respond with ...