Express - lesson 3 | HackIndia Skills

Key point

Remember this

Middleware sits between the incoming request and your route handler. It can run code, change req or res, send a response, or pass control forward.

If it does not send a response, it must call next(). Otherwise the browser, Postman, or curl keeps waiting.

Where it runs

Express has application-level middleware and router-level middleware. You use app.use() for the whole app, or router.use() for one group of routes.

In a recruiter-ready repo, middleware is useful for logging, simple checks, auth later, and parsing request bodies. This lesson focuses on flow, not body parsing.

Example

A safe logger

This logger prints the method and path, then lets the request continue. Put it before your routes.

Notice the next() call. Without it, GET /health never reaches the route.

import express from 'express';

const app = express();

function requestLogger(req, res, next) {
  console.log(`${req.method} ${req.path}`);
  next();
}

app.use(requestLogger);

app.get('/health', (req, res) => {
  res.json({ status: 'ok' });
});

app.listen(3000, () => {
  console.log('API running on port 3000');
});

Your to-do

Add one middleware

Do each one yourself, then tap it to tick it off. The ticks are only a checklist for you: they are not marked or scored.

Create a middleware function with req, res, and next.
Do one small job inside it, like logging the method and path.
Call next() if the route should still run.
Send a response if the request should stop there.
Place app.use(yourMiddleware) before the routes that need it.

0 of 5 done

Common mistake

The hanging request

The most common bug is writing middleware that neither responds nor calls next(). Your server is still running, but the request is stuck.

This often happens when you add an if check and forget the else path.

function brokenCheck(req, res, next) {
  if (req.headers['x-student-id']) {
    next();
  }
  // No response and no next() when the header is missing.
}

function fixedCheck(req, res, next) {
  if (!req.headers['x-student-id']) {
    return res.status(400).json({ error: 'x-student-id header is required' });
  }

  next();
}

Tip

Use return

When middleware sends a response, use return. It stops the function there and avoids accidentally calling next() later.

This keeps your code clear during hackathon debugging, especially when you add checks quickly.

function requireCollege(req, res, next) {
  if (!req.headers['x-college']) {
    return res.status(400).json({ error: 'x-college header is required' });
  }

  req.college = req.headers['x-college'];
  next();
}