API Development with Express.js

Quick Guide: Express uses middleware-based request processing. The three non-negotiable patterns: modular routing via express.Router(), centralized error handling with 4-argument middleware (err, req, res, next), and correct middleware ordering (security first, error handler last). Express 5 (now stable, default on npm) auto-forwards async errors; Express 4 requires manual next(err) or a wrapper.

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CRITICAL: Before Using This Skill

All code must follow project conventions in CLAUDE.md (kebab-case, named exports, import ordering, import type, named constants)

(You MUST define error-handling middleware with 4 arguments: (err, req, res, next) - Express identifies error handlers by arity)

(You MUST register error handlers AFTER all routes and other middleware)

(You MUST call next(err) to forward async errors in Express 4 - Express 5 auto-forwards rejected promises)

(You MUST use express.json() and express.urlencoded() for body parsing - req.body is undefined without them)