Centralized error handler in Express - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define an error-handling middleware in Express.

Express

app.use(function(err, req, res, [1]) {
  res.status(500).send('Something broke!');
});

Drag options to blanks, or click blank then click option'

Arequest

Bres

Cerror

Dnext

Attempts:

3 left

💡 Hint

Common Mistakes

Using only three parameters instead of four.

Naming the fourth parameter incorrectly.

The error-handling middleware in Express must have four parameters: err, req, res, and next. The next parameter is required to identify it as an error handler.

2fill in blank

medium

Complete the code to send a JSON error response with status code 400.

Express

app.use(function(err, req, res, next) {
  res.status([1]).json({ error: err.message });
});

Drag options to blanks, or click blank then click option'

A400

B404

C200

D500

Attempts:

3 left

💡 Hint

Common Mistakes

Using status 200 which means success.

Using status 404 which means not found.

Status code 400 means a bad request. Here, we send the error message as JSON with status 400.

3fill in blank

hard

Fix the error in the middleware to correctly pass the error to the next handler.

Express

app.use(function(err, req, res, next) {
  if (!err) {
    [1]();
  } else {
    res.status(500).send(err.message);
  }
});

Drag options to blanks, or click blank then click option'

Areq

Bres

Cnext

Derr

Attempts:

3 left

💡 Hint

Common Mistakes

Calling res() which is not a function.

Calling err() which is not a function.

Calling next() without arguments passes control to the next middleware. This is needed when there is no error.

4fill in blank

hard

Fill both blanks to create a middleware that logs the error message and then passes the error along.

Express

app.use(function(err, req, res, next) {
  console.[1](err.message);
  [2](err);
});

Drag options to blanks, or click blank then click option'

Alog

Bnext

Cerror

Dwarn

Attempts:

3 left

💡 Hint

Common Mistakes

Using console.error instead of console.log (both work but only one is correct here).

Calling next() without the error argument.

Use console.log to print the error message. Then call next(err) to pass the error to the next middleware.

5fill in blank

hard

Fill all three blanks to create a centralized error handler that sets status, logs error, and sends JSON response.

Express

app.use(function(err, req, res, next) {
  res.status([1]);
  console.[2](err.stack);
  res.json({ error: [3] });
});

Drag options to blanks, or click blank then click option'

A500

Berror

Cerr.message

Dwarn

Attempts:

3 left

💡 Hint

Common Mistakes

Using status 200 instead of 500.

Logging with console.log instead of console.error.

Sending the whole error object instead of just the message.

The status code 500 means server error. Use console.error to log the stack trace. Send the error message in JSON response.

Practice

1. What is the main purpose of a centralized error handler in an Express app?

easy

A. To serve static files efficiently

B. To speed up the server response time

C. To catch and handle all errors in one place

D. To manage user sessions

Solution

1. Step 1: Understand error handling in Express

   Express allows middleware to catch errors and handle them centrally.

2. Step 2: Identify the role of centralized error handler

   It collects all errors from routes and middleware to respond consistently.

3. Final Answer:

   To catch and handle all errors in one place -> Option C

4. Quick Check:

   Centralized error handler = catch all errors [OK]

Hint: Centralized error handler catches errors globally, not locally [OK]

Common Mistakes:

• Thinking it improves server speed
• Confusing with static file serving
• Mixing with session management

2. Which of the following is the correct way to define a centralized error handler middleware in Express?

easy

A. app.use((req, res) => { res.status(500).send('Error'); });

B. app.use((err, req, res, next) => { res.status(500).send(err.message); });

C. app.get('/error', (err, req, res) => { res.send('Error'); });

D. app.use((req, res, next) => { res.send('Error'); });

Solution

1. Step 1: Recall Express error handler signature

   Error handlers have four parameters: (err, req, res, next).

2. Step 2: Match the correct middleware signature

   app.use((err, req, res, next) => { res.status(500).send(err.message); }); uses all four parameters correctly and sends error message.

3. Final Answer:

   app.use((err, req, res, next) => { res.status(500).send(err.message); }); -> Option B

4. Quick Check:

   Error handler middleware needs 4 params [OK]

Hint: Error handler middleware always has 4 parameters (err, req, res, next) [OK]

Common Mistakes:

• Using only 3 parameters in error handler
• Defining error handler as route handler
• Missing the 'err' parameter

3. Given this Express code snippet, what will be the response if an error occurs in the route?

app.get('/test', (req, res, next) => {
  next(new Error('Oops!'));
});

app.use((err, req, res, next) => {
  res.status(500).send('Error caught: ' + err.message);
});

medium

A. Status 500 with body 'Error caught: Oops!'

B. Status 200 with body 'Oops!'

C. Status 404 with body 'Not Found'

D. No response, server crashes

Solution

1. Step 1: Understand error propagation in route

   The route calls next() with an Error object, triggering error middleware.

2. Step 2: Check error handler response

   The error handler sends status 500 and message including error text.

3. Final Answer:

   Status 500 with body 'Error caught: Oops!' -> Option A

4. Quick Check:

   Error passed to handler = 500 response with message [OK]

Hint: next(error) triggers centralized error handler with status 500 [OK]

Common Mistakes:

• Expecting status 200 instead of 500
• Thinking error handler is skipped
• Assuming server crashes on error

4. Identify the error in this Express app setup for centralized error handling:

app.use((req, res, next) => {
  throw new Error('Fail');
});

app.use((req, res, next) => {
  res.status(500).send('Error occurred');
});

medium

A. Error handler middleware missing 'err' parameter

B. Throwing error inside middleware is not allowed

C. Error handler middleware should be placed before routes

D. Middleware order is correct, no error

Solution

1. Step 1: Check error handler signature

   Error handlers must have 4 parameters: (err, req, res, next).

2. Step 2: Analyze given error handler

   The second middleware has only (req, res, next), so it won't catch errors.

3. Final Answer:

   Error handler middleware missing 'err' parameter -> Option A

4. Quick Check:

   Error handler needs 4 params to catch errors [OK]

Hint: Error handler must have 4 parameters to work [OK]

Common Mistakes:

• Forgetting the 'err' parameter in error handler
• Placing error handler before routes
• Thinking throwing error is disallowed

5. You want to create a centralized error handler that logs errors and sends a JSON response with status code and message. Which code snippet correctly implements this?

hard

A. app.use((req, res, next) => { console.error('Error'); res.json({ error: 'Unknown' }); });

B. app.use((err, req, res, next) => { res.send('Error occurred'); });

C. app.use((err, req, res) => { res.status(500).send('Error'); });

D. app.use((err, req, res, next) => { console.error(err); res.status(err.status || 500).json({ error: err.message }); });

Solution

1. Step 1: Confirm error handler signature and logging

   app.use((err, req, res, next) => { console.error(err); res.status(err.status || 500).json({ error: err.message }); }); uses (err, req, res, next) and logs error with console.error.

2. Step 2: Check response format and status code

   app.use((err, req, res, next) => { console.error(err); res.status(err.status || 500).json({ error: err.message }); }); sends JSON with error message and status code from error or 500.

3. Final Answer:

   app.use((err, req, res, next) => { console.error(err); res.status(err.status || 500).json({ error: err.message }); }); -> Option D

4. Quick Check:

   Proper error handler logs and sends JSON with status [OK]

Hint: Error handler needs 4 params, logs error, sends JSON with status [OK]

Common Mistakes:

• Missing 'next' parameter in error handler
• Not logging the error
• Sending plain text instead of JSON