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

Error-handling middleware signature

📖 Scenario: You are building a simple Express server that needs to handle errors gracefully. To do this, you will create an error-handling middleware function with the correct signature so Express can recognize it and use it properly.

🎯 Goal: Create an error-handling middleware function in Express with the correct four-parameter signature: err, req, res, and next. This middleware should send a 500 status code and a JSON response with an error message.

📋 What You'll Learn

Create a basic Express app instance called app

Define an error-handling middleware function with parameters err, req, res, and next

Inside the middleware, send a 500 status code and a JSON response with { error: 'Internal Server Error' }

Use app.use to register the error-handling middleware

💡 Why This Matters

🌍 Real World

Error-handling middleware is essential in Express apps to catch and respond to errors gracefully, improving user experience and debugging.

💼 Career

Knowing how to write and register error-handling middleware is a key skill for backend developers working with Express.js.

1

Create Express app instance

Write a line to import Express and create an app instance called app using express().

Express

# Import express and create app instance
# Your code here

Need a hint?

Use import express from 'express' and then const app = express().

2

Define error-handling middleware function

Write a function called errorHandler with parameters err, req, res, and next. Inside, send a 500 status code and a JSON response with { error: 'Internal Server Error' } using res.status(500).json().

Express

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

// Define errorHandler middleware function
// Your code here

Need a hint?

The error handler must have four parameters: err, req, res, and next. Use res.status(500).json({ error: 'Internal Server Error' }) inside.

3

Register error-handling middleware

Use app.use to register the errorHandler middleware function.

Express

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

function errorHandler(err, req, res, next) {
    res.status(500).json({ error: 'Internal Server Error' });
}

// Register errorHandler middleware
// Your code here

Need a hint?

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

4

Add a test route that triggers an error

Add a GET route at /error that calls next(new Error('Test error')) to trigger the error handler.

Express

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

function errorHandler(err, req, res, next) {
    res.status(500).json({ error: 'Internal Server Error' });
}

app.use(errorHandler);

// Add GET /error route to trigger error
// Your code here

Need a hint?

Use app.get('/error', (req, res, next) => { next(new Error('Test error')) }) to create the route.