Express - lesson 3 | HackIndia Skills

Express - lesson 3 | HackIndia Skills | HackIndia

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');
});

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();
}

Example

Router-level middleware

Use router-level middleware when only one group of routes needs a check. For example, project routes may require a team id, but health routes should stay open.

This helps your project structure look intentional.

import express from 'express';

const app = express();
const projectsRouter = express.Router();

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

  req.teamId = req.headers['x-team-id'];
  next();
}

projectsRouter.use(requireTeamId);

projectsRouter.get('/', (req, res) => {
  res.json({ teamId: req.teamId, projects: [] });
});

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

app.use('/projects', projectsRouter);

app.listen(3000);

Use Middleware Without Hanging Requests

Remember this

Where it runs

A safe logger

Add one middleware

The hanging request

Use return

Router-level middleware

Do this now

Quick check