Error-handling middleware in Node.js - Mini Project: Build & Apply

Error-handling middleware in Node.js

📖 Scenario: You are building a simple Node.js server using Express. You want to handle errors gracefully so users get friendly messages instead of server crashes.

🎯 Goal: Create an Express app with a route that triggers an error, then add error-handling middleware to catch and respond with a JSON error message.

📋 What You'll Learn

Create an Express app instance called app

Add a route /cause-error that throws an error with message 'Something went wrong!'

Create error-handling middleware with four parameters: err, req, res, next

In the middleware, respond with status code 500 and JSON { error: err.message }

💡 Why This Matters

🌍 Real World

Error-handling middleware is essential in real web servers to prevent crashes and provide users with clear error messages.

💼 Career

Understanding error middleware is a key skill for backend developers working with Node.js and Express.

1

Set up Express app and error route

Create an Express app instance called app by requiring express and calling it. Then add a GET route /cause-error that throws a new Error with the message 'Something went wrong!'.

Node.js

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

// Your code here

Hint

Use const app = express() to create the app. Use app.get('/cause-error', (req, res) => { throw new Error('Something went wrong!'); }) to add the route.

2

Add a helper variable for the error status code

Create a constant called errorStatus and set it to 500. This will be used as the HTTP status code for errors.

Node.js

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

app.get('/cause-error', (req, res) => {
    throw new Error('Something went wrong!');
});

// Your code here

Hint

Write const errorStatus = 500; below the route.

3

Create error-handling middleware

Add error-handling middleware to the app using app.use. The middleware function must have four parameters: err, req, res, next. Inside, respond with status code errorStatus and send JSON with the key error and value err.message.

Node.js

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

app.get('/cause-error', (req, res) => {
    throw new Error('Something went wrong!');
});

const errorStatus = 500;

// Your code here

Hint

Use app.use with a function that has err, req, res, next parameters. Inside, call res.status(errorStatus).json({ error: err.message }).

4

Start the server to listen on port 3000

Add code to make the app listen on port 3000 using app.listen. Provide a callback function that does nothing (empty arrow function).

Node.js

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

app.get('/cause-error', (req, res) => {
    throw new Error('Something went wrong!');
});

const errorStatus = 500;

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

// Your code here

Hint

Use app.listen(3000, () => {}) to start the server.

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