Error-handling middleware in Express - Mini Project: Build & Apply

Error-handling middleware

📖 Scenario: You are building a simple Express server that handles user requests. Sometimes errors happen, like when a user asks for a page that does not exist. To keep your server friendly and clear, you want to add a special error-handling middleware that catches errors and sends a nice message back to the user.

🎯 Goal: Create an Express app with a route that triggers an error, then add an error-handling middleware that catches the error and sends a JSON response with the error message and a 500 status code.

📋 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 an error-handling middleware function with four parameters: err, req, res, next

In the error handler, send a JSON response with status 500 and message from the error

Use app.use to add the error-handling middleware after the route

💡 Why This Matters

🌍 Real World

Error-handling middleware helps keep web servers stable and user-friendly by catching unexpected problems and sending clear messages.

💼 Career

Understanding error-handling middleware is essential for backend developers working with Express to build reliable and maintainable web applications.

1

Create the Express app and route

Write code to import Express, create an app instance called app, and add a GET route /cause-error that throws a new Error with the message 'Something went wrong!'.

Express

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

// Your code here

Need a hint?

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

2

Create the error-handling middleware function

Write a function called errorHandler that takes four parameters: err, req, res, and next. Inside the function, send a JSON response with status code 500 and a message property set to err.message.

Express

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

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

// Write errorHandler function below
// Your code here

Need a hint?

The error handler must have four parameters. Use res.status(500).json({ message: err.message }) to send the error message.

3

Add the error-handling middleware to the app

Use app.use to add the errorHandler middleware to the Express app after the route definition.

Express

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

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

function errorHandler(err, req, res, next) {
    res.status(500).json({ message: err.message });
}

// Add errorHandler middleware below
// Your code here

Need a hint?

Use app.use(errorHandler) to add the error handler after all routes.

4

Start the Express server

Add code to start the Express server on port 3000 using app.listen. Inside the listen callback, log the message 'Server running on port 3000'.

Express

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

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

function errorHandler(err, req, res, next) {
    res.status(500).json({ message: err.message });
}

app.use(errorHandler);

// Start server on port 3000
// Your code here

Need a hint?

Use app.listen(3000, () => { console.log('Server running on port 3000') }) to start the server.