Express.js and Middleware

Build production-style APIs with Express.js, routing, middleware, validation, centralized errors, and request lifecycle management.

Express Server and Routing

Express is a lightweight Node.js web framework that provides routing, middleware, request handling, and response helpers. It is commonly used to build REST APIs and web servers.

Routes map HTTP methods and URL patterns to handler functions.

Express REST Routes

import express from 'express';

const app = express();
app.use(express.json());

app.get('/users', (req, res) => {
  res.json([{ id: 1, name: 'Suriya' }]);
});

app.post('/users', (req, res) => {
  res.status(201).json(req.body);
});

app.listen(3000);

💡 The application parses JSON request bodies and defines GET and POST endpoints.

Key Takeaways

Express simplifies HTTP routing and middleware composition.
Use routers to keep larger applications modular.
Validate incoming request data instead of trusting client input.

Knowledge Check

Which Express middleware parses JSON request bodies?

Understanding Middleware

Express middleware functions receive req, res, and next. Middleware can inspect or modify requests, perform authentication, log requests, validate input, and terminate responses.

Middleware executes in registration order. Calling next() passes control to the next matching middleware or route handler.

Express Middleware Chain

Request Logging Middleware

function logger(req, res, next) {
  const start = Date.now();
  res.on('finish', () => {
    console.log(req.method, req.originalUrl, Date.now() - start, 'ms');
  });
  next();
}

app.use(logger);

💡 The middleware records the request start time and logs the response duration when the response finishes.

Key Takeaways

Middleware runs in registration order.
Call next() when middleware does not end the response.
Middleware is useful for cross-cutting concerns.

Knowledge Check

What happens when middleware calls `next()`?

Centralized Error Handling

Express applications should use centralized error handling instead of duplicating response logic throughout controllers. Error middleware has the signature (err, req, res, next).

Production systems should avoid exposing internal stack traces or secrets to clients. Errors should be logged with sufficient context while API responses contain safe information.

Do Not Leak Internal Errors

Never return database connection strings, access tokens, stack traces, or internal implementation details to untrusted clients.

Error Middleware

app.use((err, req, res, next) => {
  console.error(err);

  if (res.headersSent) {
    return next(err);
  }

  res.status(err.statusCode || 500).json({
    error: err.publicMessage || 'Internal server error'
  });
});

💡 The error middleware logs the internal error and returns a controlled response to the client.

Key Takeaways

Centralize API error responses.
Separate internal error details from public API messages.
Handle errors consistently across asynchronous operations.

Knowledge Check

What distinguishes Express error-handling middleware?

Express Server and Routing

Understanding Middleware

Centralized Error Handling

STAY UPDATED WITH BYTES!