One of the first things any web developer learns is how to build an API. Express makes this straightforward, but there are patterns worth adopting early.
Start by separating your routes from your business logic. Keep your route handlers thin — they should parse the request, call a service, and return the response. All the real work belongs in service functions or model methods.
For MongoDB specifically, Mongoose schemas give you validation at the application layer. Define your schemas carefully: use required fields, enums for constrained values, and default values where it makes sense.
Error handling is another area where small investments pay off. A centralized error handler middleware catches everything that falls through, and custom error classes let you throw meaningful errors from anywhere in the stack.
Finally, think about your response format early. Consistent JSON structures — always including a status, data payload, and optional message — make your API predictable for consumers.