Basics

An Express app is a list of routes and a call to listen(). Answer the cats as JSON, one cat by its id, a 404 for a missing one, and a name as plain text rather than the HTML Express assumes for a string.

Read request bodies with express.json(), create, change and delete cats, and declare the routes one path shares once with app.route(). Plus what Express 5 leaves in req.body when there is nothing to parse.

Express is a chain of functions, and a route is only the last link. Write middleware that numbers every request, logs each one with its final status, and guards two routes with an API key, and see why the order they are mounted in decides what happens.

Split the app into routers, one per resource and one file each, mounted under their paths. Load a cat once for every route with :id, nest an owner's cats under the owner, and see what the request knows about where it is.

The request and the response in detail: answer JSON or CSV with res.format(), refuse a body that is not JSON with req.is(), remember a preference in a cookie with cookie-parser, and redirect to a resource that can move.

Let routes throw and answer every error in one place: an error class that carries its status, a 404 for anything no route matched, body-parser's error for broken JSON, Express 5 catching async handlers, and a 500 that tells the client nothing about your internals.