When building API endpoints with Express, make request lifecycle control-flow and response semantics explicit and consistent: - Middleware chaining: only call `next()` when you intend to continue to the next middleware/handler; if you don’t call `next()`, the request-response cycle will stop. - Ordering: rely on the fact middleware executes in the order it’s added; mount router modules intentionally. - Response outcomes: in each terminal handler, return an HTTP status code that matches the outcome (e.g., `400` for bad requests, `200` for success, `500` for server errors) and use an appropriate body format (string vs JSON). Example: ```js const express = require('express'); const app = express(); // Global middleware (runs in order) app.use((req, res, next) => { console.log(req.method, req.url); next(); // continue the lifecycle }); const api = express.Router(); api.use('/v1', (req, res, next) => { // router-level middleware next(); }); api.get('/v1/resource', (req, res) => { // terminal handler: choose status code + body that match the outcome return res.status(200).json({ ok: true }); }); api.post('/v1/resource', express.json(), (req, res) => { if (!req.body || !req.body.name) { return res.status(400).send('Bad Request'); } return res.status(201).json({ created: true }); }); app.use(api); app.listen(3000); ``` Use this checklist in reviews: 1) Does every middleware either call `next()` or send a response (intentionally terminating)? 2) Are middleware/routers mounted in the intended order/module boundaries? 3) Do your route handlers return status codes and response bodies that accurately represent success/client error/server error?