Conditional middleware execution in Express - Mini Project: Build & Apply

Conditional middleware execution in Express

📖 Scenario: You are building a simple Express server that handles requests differently based on a condition. Sometimes, you want to run a middleware only if a certain query parameter is present.

🎯 Goal: Create an Express server with a middleware that runs only when the request has a query parameter runMiddleware=true. Otherwise, the middleware should be skipped.

📋 What You'll Learn

Create an Express app with a GET route at /check

Create a middleware function called conditionalMiddleware

Use a variable shouldRun to check if the query parameter runMiddleware equals true

Apply the middleware only when shouldRun is true

Send a response with text Middleware ran if middleware ran, else Middleware skipped

💡 Why This Matters

🌍 Real World

Conditional middleware is useful when you want to apply certain logic only for specific requests, like authentication or feature toggles.

💼 Career

Backend developers often need to control middleware execution based on request data to build flexible and efficient APIs.

1

Set up Express app and route

Create an Express app by requiring express and calling express(). Then create a GET route at /check that sends the text Middleware skipped as response.

Express

const express = require('express');
const app = express();

// Create GET route at /check
// Your code here

Hint

Use app.get('/check', (req, res) => { ... }) to create the route.

2

Add a variable to check query parameter

Inside the GET route at /check, create a variable called shouldRun that is true if req.query.runMiddleware equals the string 'true', otherwise false.

Express

const express = require('express');
const app = express();

app.get('/check', (req, res) => {
  // Create shouldRun variable to check if req.query.runMiddleware === 'true'
  // Your code here
  res.send('Middleware skipped');
});

Hint

Use const shouldRun = req.query.runMiddleware === 'true' inside the route handler.

3

Create the conditional middleware function

Create a middleware function called conditionalMiddleware that takes req, res, and next as parameters. Inside it, send the response text Middleware ran. Do not call next() because this middleware ends the response.

Express

const express = require('express');
const app = express();

// Create conditionalMiddleware function
// Your code here

app.get('/check', (req, res) => {
  const shouldRun = req.query.runMiddleware === 'true';
  res.send('Middleware skipped');
});

Hint

Define function conditionalMiddleware(req, res, next) { res.send('Middleware ran') }.

4

Use the middleware conditionally in the route

Modify the GET route at /check to run conditionalMiddleware only if shouldRun is true. If shouldRun is false, send Middleware skipped as response. Use an if statement inside the route handler to decide.

Express

const express = require('express');
const app = express();

function conditionalMiddleware(req, res, next) {
  res.send('Middleware ran');
}

app.get('/check', (req, res) => {
  const shouldRun = req.query.runMiddleware === 'true';
  // Run conditionalMiddleware if shouldRun is true, else send 'Middleware skipped'
  // Your code here
});

Hint

Use if (shouldRun) { conditionalMiddleware(req, res, () => {}) } else { res.send('Middleware skipped') }.

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