Error-handling middleware in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is error-handling middleware in Node.js Express?

It is a special type of middleware designed to catch and handle errors that occur during request processing. It has four parameters: (err, req, res, next).

beginner

How do you define an error-handling middleware function in Express?

By creating a function with four arguments: (err, req, res, next). Express recognizes it as error-handling middleware because of the first 'err' parameter.

intermediate

Why should error-handling middleware be placed after all other middleware and routes?

Because it catches errors from previous middleware or routes. If placed earlier, it won't catch errors from later code.

intermediate

What happens if you call next() with an error inside a middleware?

Express skips normal middleware and routes, then calls the next error-handling middleware to handle the error.

beginner

How can you send a user-friendly error message from error-handling middleware?

Use res.status() to set HTTP status code and res.json() or res.send() to send a clear message explaining the error.

Which of these is the correct signature for error-handling middleware in Express?

A (req, res)

B (err, req, res, next)

C (req, res, next)

D (err, req, res)

Error-handling middleware must have four parameters: err, req, res, and next.

Where should error-handling middleware be placed in your Express app?

A Only inside route handlers

B Before all routes and middleware

C Anywhere in the code

D After all routes and middleware

It should be placed after all routes and middleware to catch errors from them.

What does calling next(err) inside a middleware do?

A Passes control to the next error-handling middleware

B Sends a response immediately

C Stops the request processing

D Passes control to the next normal middleware

Calling next with an error skips normal middleware and goes to error-handling middleware.

How can you send a 404 error using error-handling middleware?

A next(404)

B res.send(404)

C res.status(404).send('Not Found')

D throw 404

Use res.status(404) to set the status code and send a message.

What parameter identifies a middleware as error-handling middleware?

A The presence of 'err' as the first parameter

B The use of res.send()

C The use of next()

D The absence of next parameter

Error-handling middleware has four parameters, starting with 'err'.

Explain how error-handling middleware works in Express and why its signature is important.

Think about how Express knows which middleware handles errors.

Describe the steps to create and use error-handling middleware in a Node.js Express app.

Focus on function signature, placement, and usage.

Practice

1. What is the correct number of parameters for an error-handling middleware function in Node.js Express?

easy

A. One parameter: err

B. Three parameters: req, res, next

C. Two parameters: req, res

D. Four parameters: err, req, res, next

Solution

1. Step 1: Recall the error middleware signature

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

2. Step 2: Understand the role of each parameter

   The first parameter is the error object, followed by the request, response, and next function to pass control.

3. Final Answer:

   Four parameters: err, req, res, next -> Option D

4. Quick Check:

   Error middleware = 4 parameters [OK]

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

Common Mistakes:

• Using 3 parameters like normal middleware
• Forgetting the err parameter
• Placing error middleware before routes

2. Which of the following is the correct syntax to define an error-handling middleware in Express?

easy

A. app.use((err, req, res, next) => { /* handle error */ })

B. app.use((req, res, next, err) => { /* handle error */ })

C. app.use((err, req, next, res) => { /* handle error */ })

D. app.use((req, err, res, next) => { /* handle error */ })

Solution

1. Step 1: Check parameter order in error middleware

   The correct order is err, req, res, next for error-handling middleware.

2. Step 2: Verify the syntax matches Express pattern

   app.use((err, req, res, next) => { /* handle error */ }) matches the correct order and syntax for error middleware.

3. Final Answer:

   app.use((err, req, res, next) => { /* handle error */ }) -> Option A

4. Quick Check:

   Correct parameter order = err, req, res, next [OK]

Hint: Error middleware params start with err, then req, res, next [OK]

Common Mistakes:

• Mixing parameter order
• Placing err parameter last or in the middle
• Using normal middleware syntax for error middleware

3. Consider this Express app code snippet:

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

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

What will the client receive when requesting /test?

medium

A. A 404 Not Found error

B. A 500 status with 'Error caught' message

C. A 200 status with no response body

D. The server crashes with an uncaught exception

Solution

1. Step 1: Understand error thrown in route handler

   The route throws an error which Express catches and passes to error middleware.

2. Step 2: Error middleware sends 500 status and message

   The error middleware sets status 500 and sends 'Error caught' to the client.

3. Final Answer:

   A 500 status with 'Error caught' message -> Option B

4. Quick Check:

   Error thrown -> error middleware response [OK]

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

Common Mistakes:

• Expecting 200 OK despite error
• Thinking server crashes without middleware
• Confusing 404 with error middleware response

4. What is wrong with this error-handling middleware code?

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

medium

A. It should have only two parameters

B. It should be placed before routes

C. It is missing the next parameter

D. It should not use res.status

Solution

1. Step 1: Check error middleware parameters

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

2. Step 2: Identify missing parameter

   The code is missing the next parameter, which is required even if unused.

3. Final Answer:

   It is missing the next parameter -> Option C

4. Quick Check:

   Error middleware needs 4 params including next [OK]

Hint: Error middleware always needs 4 parameters, including next [OK]

Common Mistakes:

• Omitting next parameter
• Placing error middleware before routes
• Using wrong parameter order

5. You want to log errors and then send a response in your Express app. Which is the best way to write the error-handling middleware?

app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).send('Something broke!');
});

What is a potential problem with this code if you add more error middleware after it?

hard

A. It does not call next(), so later error middleware won't run

B. It calls next() without an error, causing infinite loop

C. It sends response twice causing an error

D. It should not log errors in middleware

Solution

1. Step 1: Analyze middleware flow after sending response

   Once res.send is called, response is sent and middleware usually ends.

2. Step 2: Understand next() usage in error middleware

   If next() is not called, any following error middleware won't execute.

3. Step 3: Identify problem if more error middleware exists

   Without calling next(err), later error middleware is skipped, possibly missing logging or handling.

4. Final Answer:

   It does not call next(), so later error middleware won't run -> Option A

5. Quick Check:

   Missing next() call skips later error middleware [OK]

Hint: Call next(err) to continue error middleware chain [OK]

Common Mistakes:

• Not calling next() after sending response
• Calling next() without error causing loops
• Logging errors outside middleware