Conditional middleware execution in Express

Introduction

Sometimes you want middleware to run only for certain requests, not all. Conditional middleware helps you control when middleware runs.

Run authentication only on protected routes.

Log requests only for specific URL paths.

Apply rate limiting only for API endpoints.

Skip middleware for static files like images or CSS.

Syntax

Express

app.use((req, res, next) => {
  if (condition) {
    middlewareFunction(req, res, next);
  } else {
    next();
  }
});

The next() function passes control to the next middleware.

Check your condition inside the middleware to decide whether to run the middleware logic or skip it.

Examples

This middleware logs only requests starting with '/api'.

Express

app.use((req, res, next) => {
  if (req.path.startsWith('/api')) {
    console.log('API request');
  }
  next();
});

This middleware runs only for POST requests.

Express

app.use((req, res, next) => {
  if (req.method === 'POST') {
    console.log('POST request received');
  }
  next();
});

This runs authentication only on routes starting with '/private'.

Express

const authMiddleware = (req, res, next) => {
  if (req.headers.authorization) {
    next();
  } else {
    res.status(401).send('Unauthorized');
  }
};

app.use((req, res, next) => {
  if (req.path.startsWith('/private')) {
    authMiddleware(req, res, next);
  } else {
    next();
  }
});

Sample Program

This Express app uses conditional middleware to log only requests to paths starting with '/api'. Other requests skip the log.

Express

import express from 'express';

const app = express();

// Middleware that logs only API requests
app.use((req, res, next) => {
  if (req.path.startsWith('/api')) {
    console.log(`API request to ${req.path}`);
  }
  next();
});

// Simple route
app.get('/api/data', (req, res) => {
  res.send('Here is your API data');
});

app.get('/', (req, res) => {
  res.send('Welcome to the homepage');
});

app.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});

Important Notes

Always call next() to avoid hanging requests.

Conditions can check path, method, headers, or any request property.

Use this pattern to keep middleware efficient and avoid unnecessary work.

Summary

Conditional middleware runs only when you want it to.

Use if checks inside middleware to decide execution.

Always call next() to continue request handling.

Practice

1. What is the main purpose of conditional middleware in Express?

easy

A. To run middleware only when certain conditions are met

B. To run all middleware for every request without exception

C. To replace route handlers completely

D. To stop the server from accepting requests

Solution

1. Step 1: Understand middleware role

   Middleware functions process requests and can modify them or respond.

2. Step 2: Identify conditional middleware purpose

   Conditional middleware runs only when a specific condition is true, saving resources and controlling flow.

3. Final Answer:

   To run middleware only when certain conditions are met -> Option A

4. Quick Check:

   Conditional middleware = run only if condition true [OK]

Hint: Think: middleware runs only if condition matches [OK]

Common Mistakes:

• Assuming middleware always runs for every request
• Confusing middleware with route handlers
• Thinking middleware stops server requests

2. Which of the following is the correct way to conditionally execute middleware based on the request path?

easy

A. app.use((req, res, next) => { if (req.path === '/admin') res.send('OK'); });

B. app.use((req, res, next) => { if (req.method === 'POST') res.send('Allowed'); else next(); });

C. app.use((req, res, next) => { if (req.path = '/admin') next(); else next(); });

D. app.use((req, res, next) => { if (req.path === '/admin') next(); else res.send('No access'); });

Solution

1. Step 1: Check condition syntax

   app.use((req, res, next) => { if (req.path === '/admin') next(); else res.send('No access'); }); correctly uses '===' to compare req.path to '/admin'.

2. Step 2: Verify middleware flow

   app.use((req, res, next) => { if (req.path === '/admin') next(); else res.send('No access'); }); calls next() only if path matches; otherwise, it sends a response, stopping further middleware.

3. Final Answer:

   app.use((req, res, next) => { if (req.path === '/admin') next(); else res.send('No access'); }); -> Option D

4. Quick Check:

   Correct condition and flow = app.use((req, res, next) => { if (req.path === '/admin') next(); else res.send('No access'); }); [OK]

Hint: Use '===' for comparison and call next() properly [OK]

Common Mistakes:

• Using single '=' instead of '===' for comparison
• Not calling next() to continue middleware chain
• Sending response without ending middleware properly

3. Consider this middleware code snippet:

app.use((req, res, next) => {
  if (req.query.token === '123') {
    console.log('Token valid');
    next();
  } else {
    res.status(401).send('Unauthorized');
  }
});
app.get('/', (req, res) => res.send('Welcome!'));

What will be the output when a GET request is made to '/' with query ?token=123?

medium

A. Console logs 'Token valid' but response is 'Unauthorized'

B. Console logs 'Token valid' and response is 'Welcome!'

C. No response, request hangs

D. Response is 'Unauthorized' with status 401

Solution

1. Step 1: Check token condition

   Request query token is '123', so condition is true and 'Token valid' logs.

2. Step 2: Middleware calls next()

   Calling next() passes control to next handler, which sends 'Welcome!'.

3. Final Answer:

   Console logs 'Token valid' and response is 'Welcome!' -> Option B

4. Quick Check:

   Valid token triggers next() and welcome message [OK]

Hint: If condition true, next() runs next handler [OK]

Common Mistakes:

• Assuming response is 'Unauthorized' despite valid token
• Thinking middleware always sends response
• Missing that next() continues to route handler

4. Identify the error in this conditional middleware:

app.use((req, res, next) => {
  if (req.headers['x-auth']) {
    next();
  }
  res.status(403).send('Forbidden');
});

medium

A. Middleware does not check request method

B. next() is called without arguments

C. Missing else block causes response sent twice

D. Headers property is misspelled

Solution

1. Step 1: Analyze middleware flow

   If header exists, next() is called but code continues to send 403 response anyway.

2. Step 2: Identify missing else

   Without else, response is sent after next(), causing errors or double responses.

3. Final Answer:

   Missing else block causes response sent twice -> Option C

4. Quick Check:

   Always use else or return after next() [OK]

Hint: Use else or return after next() to avoid double response [OK]

Common Mistakes:

• Not using else or return after next()
• Assuming next() stops code execution
• Ignoring middleware flow after next()

5. You want to create middleware that only runs for POST requests to '/submit' and logs 'Processing submission'. Which code correctly implements this conditional middleware?

hard

A. app.use((req, res, next) => { if (req.path === '/submit' && req.method === 'POST') { console.log('Processing submission'); } next(); });

B. app.use('/submit', (req, res, next) => { if (req.method === 'POST') { console.log('Processing submission'); } next(); });

C. app.post('/submit', (req, res, next) => { console.log('Processing submission'); next(); });

D. app.use('/submit', (req, res, next) => { if (req.method = 'POST') { console.log('Processing submission'); next(); } });

Solution

1. Step 1: Check condition for path and method

   app.use((req, res, next) => { if (req.path === '/submit' && req.method === 'POST') { console.log('Processing submission'); } next(); }); correctly checks both req.path and req.method inside middleware.

2. Step 2: Confirm middleware calls next()

   app.use((req, res, next) => { if (req.path === '/submit' && req.method === 'POST') { console.log('Processing submission'); } next(); }); calls next() after logging, ensuring request continues.

3. Step 3: Evaluate other options

   app.use('/submit', (req, res, next) => { if (req.method === 'POST') { console.log('Processing submission'); } next(); }); runs on all methods for '/submit', app.post('/submit', (req, res, next) => { console.log('Processing submission'); next(); }); is a route handler, app.use('/submit', (req, res, next) => { if (req.method = 'POST') { console.log('Processing submission'); next(); } }); uses '=' instead of '===' causing assignment error.

4. Final Answer:

   app.use((req, res, next) => { if (req.path === '/submit' && req.method === 'POST') { console.log('Processing submission'); } next(); }); -> Option A

5. Quick Check:

   Check both path and method, call next() [OK]

Hint: Check both path and method inside middleware, then call next() [OK]

Common Mistakes:

• Using assignment '=' instead of comparison '==='
• Not calling next() after middleware logic
• Confusing route handlers with middleware