Express/Fastify Backend Patterns

When to use this skill

Use this skill when building Node.js HTTP APIs with Express or Fastify and you want safe request validation, predictable error handling, and maintainable routing/service layering.

Core patterns

Boundary validation

Validate at the edge (request body/params/query):

• reject invalid shapes early
• normalize types (numbers, booleans)
• avoid passing raw request objects into services

Layering

Keep:

• routes/controllers: parsing + HTTP response
• services: business logic
• repositories/adapters: DB/external APIs

Error handling

Rules:

• throw domain errors in services
• map to HTTP status codes in one place (middleware/error handler)
• include request IDs in logs

Timeouts and retries

• apply timeouts to outbound HTTP/DB calls
• retries must be bounded and idempotent

Observability

• structured logs (no secrets)
• basic metrics for request duration/error rates (if available)

Atlarix tool notes

• Explore: locate route registration and middleware order with grep/glob.
• Build: make small edits; run API tests; keep handlers thin.
• Debug: capture full error + stack; reproduce with a single curl/HTTP request.
• Review: check validation, authz, and that errors don’t leak secrets.

Common mistakes to avoid

• No validation (passing raw JSON deep into code)
• Controllers doing business logic
• Swallowing errors in catch blocks
• Missing timeouts on outbound calls

Mini-checklist

• Validation at boundaries
• Centralized error mapping
• Clear layering
• Timeouts on outbound calls