Error-handling middleware in Node.js

Introduction

Error-handling middleware helps catch and respond to errors in your app. It keeps your app from crashing and shows friendly messages.

When you want to catch errors from routes or other middleware

When you want to send a clear error message to users

When you want to log errors for debugging

When you want to handle unexpected problems gracefully

When you want to keep your app running even if something goes wrong

Syntax

Node.js

app.use(function (err, req, res, next) {
  // handle the error here
  res.status(500).send('Something broke!');
});

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

It must be added after all other middleware and routes.

Examples

Logs the error stack and sends a simple error message to the client.

Node.js

app.use(function (err, req, res, next) {
  console.error(err.stack);
  res.status(500).send('Oops! Something went wrong.');
});

Handles different error types with custom messages.

Node.js

app.use(function (err, req, res, next) {
  if (err.type === 'database') {
    res.status(500).send('Database error occurred');
  } else {
    res.status(500).send('General error');
  }
});

Sample Program

This app throws an error when visiting the home page. The error-handling middleware catches it, logs it, and sends a friendly message.

Node.js

import express from 'express';
const app = express();

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

// Error-handling middleware
app.use((err, req, res, next) => {
  console.error('Error caught:', err.message);
  res.status(500).send('Sorry, an error happened!');
});

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

Important Notes

Always place error-handling middleware last, after all routes.

Use console.error to log errors for debugging.

Sending generic messages helps keep your app secure by not revealing details.

Summary

Error-handling middleware catches errors in your app.

It has four parameters: err, req, res, and next.

Place it after all other middleware and routes to work properly.

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