Error-handling middleware 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], next) {
  res.status(500).send('Something broke!');
});

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

Ares

Berror

Cnext

Drequest

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'res' or 'request' as the fourth parameter instead of 'next'.

Omitting the fourth parameter entirely.

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

2fill in blank

medium

Complete the code to send a JSON error response with status 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

B200

C404

D500

Attempts:

3 left

💡 Hint

Common Mistakes

Using 200 which means success.

Using 500 which means server error.

To indicate a client error, the status code 400 is used. This sends a JSON response with the error message.

3fill in blank

hard

Fix the error in the middleware signature to correctly handle errors.

Express

app.use(function(req, res, next, [1]) {
  res.status(500).send('Error occurred');
});

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

Arequest

Berror

Cnext

Derr

Attempts:

3 left

💡 Hint

Common Mistakes

Placing the error parameter last instead of first.

Using wrong parameter names or order.

The error-handling middleware must have the error parameter err as the first argument, followed by req, res, and next. The order matters.

4fill in blank

hard

Fill both blanks to correctly pass the error to the next middleware.

Express

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

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

Anext

Bres

Csend

Derror

Attempts:

3 left

💡 Hint

Common Mistakes

Using res or send instead of next.

Not calling next with the error.

The next function is used to pass control to the next middleware. Calling next(err) passes the error along the middleware chain.

5fill in blank

hard

Fill all three blanks to create a middleware that logs the error and sends a 500 response.

Express

app.use(function([1], req, res, [2]) {
  console.error([3]);
  res.status(500).send('Server error');
});

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

Aerr

Bnext

Derror

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up parameter names.

Not logging the error correctly.

The error parameter is named err and the next middleware function is next. Logging the error uses console.error(err).

Practice

1. What is the main purpose of error-handling middleware in Express?

easy

A. To serve static files like images and CSS

B. To parse incoming JSON request bodies

C. To catch errors and prevent the server from crashing

D. To log every request made to the server

Solution

1. Step 1: Understand middleware roles

   Middleware can serve many roles, but error-handling middleware specifically catches errors.

2. Step 2: Identify error-handling middleware purpose

   Error-handling middleware prevents the server from crashing by catching errors and responding properly.

3. Final Answer:

   To catch errors and prevent the server from crashing -> Option C

4. Quick Check:

   Error-handling middleware = catch errors [OK]

Hint: Error middleware catches errors to keep server running [OK]

Common Mistakes:

• Confusing error middleware with static file serving
• Thinking it parses request bodies
• Assuming it logs requests

2. Which of the following is the correct signature for an Express error-handling middleware function?

easy

A. (err, req, res, next)

B. (req, res, next)

C. (req, res)

D. (err, res, next)

Solution

1. Step 1: Recall error middleware parameters

   Error-handling middleware always has four parameters: err, req, res, next.

2. Step 2: Match the correct parameter order

   Only (err, req, res, next) has all four parameters in the correct order.

3. Final Answer:

   (err, req, res, next) -> Option A

4. Quick Check:

   Error middleware signature = (err, req, res, next) [OK]

Hint: Error middleware always has 4 parameters starting with err [OK]

Common Mistakes:

• Using only three parameters
• Omitting the err parameter
• Wrong parameter order

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

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

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

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

app.listen(3000);

medium

A. The client receives 'Error caught: Oops!' with status 500

B. The server crashes and stops running

C. The client receives a 404 Not Found error

D. The client receives an empty response with status 200

Solution

1. Step 1: Identify error thrown in route

   The route throws an error with message 'Oops!'.

2. Step 2: Check error-handling middleware response

   The error middleware catches the error and sends a 500 status with message including 'Oops!'.

3. Final Answer:

   The client receives 'Error caught: Oops!' with status 500 -> Option A

4. Quick Check:

   Error thrown = caught and sent with 500 [OK]

Hint: Thrown errors go to error middleware response [OK]

Common Mistakes:

• Expecting server crash on error
• Confusing 404 with error response
• Missing error message in response

4. What is wrong with the following error-handling middleware?

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

medium

A. It should be placed before routes

B. The parameters are in the wrong order

C. It is missing the next() call

D. It should not have four parameters

Solution

1. Step 1: Check parameter order for error middleware

   Error middleware must have parameters in order: err, req, res, next.

2. Step 2: Identify incorrect order in code

   The code has (req, res, next, err) which is incorrect.

3. Final Answer:

   The parameters are in the wrong order -> Option B

4. Quick Check:

   Error middleware params order = err, req, res, next [OK]

Hint: Error middleware params start with err, not req [OK]

Common Mistakes:

• Swapping err and req parameters
• Placing error middleware before routes
• Forgetting to call next() when needed

5. You want to create an error-handling middleware that logs the error message and then passes the error to the next middleware. Which of the following implementations is correct?

A) app.use((err, req, res, next) => {
  console.log(err.message);
  next(err);
});

B) app.use((req, res, next, err) => {
  console.log(err.message);
  next();
});

C) app.use((err, req, res) => {
  console.log(err.message);
  next(err);
});

D) app.use((err, req, res, next) => {
  console.log(err.message);
  res.send('Error logged');
});

hard

A. app.use((err, req, res, next) => {
console.log(err.message);
res.send('Error logged');
});

B. app.use((req, res, next, err) => {
console.log(err.message);
next();
});

C. app.use((err, req, res) => {
console.log(err.message);
next(err);
});

D. app.use((err, req, res, next) => {
console.log(err.message);
next(err);
});

Solution

1. Step 1: Recall requirements for error-handling middleware that chains

   It must have four parameters starting with err, log err.message, and call next(err) to pass the error to the next handler.

2. Step 2: Match against provided implementations

   Implementations with incorrect parameter order or count fail to receive or handle err properly; those that respond instead of calling next(err) break the chain.

3. Final Answer:

   app.use((err, req, res, next) => {
     console.log(err.message);
     next(err);
   }); -> Option D

4. Quick Check:

   4 params + next(err) = correct chaining [OK]

Hint: Error middleware must have 4 params and call next(err) to continue [OK]

Common Mistakes:

• Wrong parameter order
• Forgetting next parameter
• Not calling next(err) to pass error along
• Sending response too early