Checkpoint: API Mechanic

This checkpoint rebuilds the backend's moving parts in runnable form: the router you wrote by hand, the pipeline, and the validation posture — graded as pure logic, the same reasoning Express automates.

The server is just functions

Node's HTTP server is a function from request to response, and everything you practiced — routing, middleware, validation, error shapes — is composition of that function. The checkpoint strips the framework away one more time so the pattern is yours, not Express's. When you later meet Express, Fastify, or the route handlers of a full framework, you will recognize old friends.

From handlers to habits

The checkpoint's scenarios compress what a real service does every request: read the request safely, decide what it means, validate that meaning, touch state, and answer with an honest status code. When you later add a framework, these are still the moves — Express merely gives them friendlier syntax. Passing here means the pattern, not the API, belongs to you.