[Express] Basic concept notes

Middleware (handlers)

• Order-sensitive
• Triggered by previous MW next()
• Every middleware should call next() or end the request-response cycle
• Changes to req and res will be passed down
• App level MW: Bind to app
• Router level MW: Bind to express.Router()
  • next('route'): Bypass remaining route callbacks
  • next('router'): Bypass remaining routes
• Error-handling MW: (err, req, res, next) => { … }

Error handling

• When an error occurs with next(myErr), remaining non-error MW will be skipped
• Synchronous code: throw error;
• Asynchronous code
  • Express 4: next(error);
  • Express 5: automatically call next(error);