Middleware Guide

This skill explains how to structure and apply Express middleware safely and consistently.

Types of middleware

• •Auth: authenticates identity and attaches it to the request context
• •Authorization: enforces permissions/roles
• •Validation: validates input and short-circuits on invalid requests
• •Context: attaches correlation ids, request metadata, and logging context
• •Error boundary: catches errors and converts them into consistent responses

Recommended structure

src/middleware/
  auth.ts
  authorization.ts
  request_context.ts
  validation.ts
  error_boundary.ts

Error boundary pattern

Ensure async errors are captured:

export function asyncHandler(fn) {
  return (req, res, next) => Promise.resolve(fn(req, res, next)).catch(next);
}

Register error middleware last:

app.use("/api", routes);
app.use(errorBoundary); // last

Related Skills

• •architecture-overview
• •async-and-errors