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

Need a 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

Need a 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

Need a 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

Need a hint?

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