Error response formatting in Express - Mini Project: Build & Apply

Error Response Formatting in Express

📖 Scenario: You are building a simple Express server that handles user requests. When something goes wrong, you want to send back a clear and consistent error message in JSON format.

🎯 Goal: Create an Express server that sends error responses with a JSON object containing error and message fields.

📋 What You'll Learn

Create an Express app instance

Add a route /user that triggers an error

Create a configuration variable errorMessage with the text 'User not found'

Use Express error handling middleware to send a JSON error response with error and message

Ensure the error response has HTTP status code 404

💡 Why This Matters

🌍 Real World

Web servers often need to send clear error messages in JSON format so client apps can handle errors gracefully.

💼 Career

Knowing how to format error responses in Express is essential for backend developers building APIs.

1

Setup Express app and route

Create an Express app by requiring express and calling express(). Then add a GET route /user that calls next(new Error('User not found')) to simulate an error.

Express

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

// Your code here

Need a hint?

Remember to require Express and create the app instance first. Then add a GET route for '/user' that triggers an error using next(new Error('User not found')).

2

Add error message configuration

Create a constant variable called errorMessage and set it to the string 'User not found'.

Express

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

app.get('/user', (req, res, next) => {
    next(new Error('User not found'));
});

// Your code here

Need a hint?

Define a constant named errorMessage and assign it the exact string 'User not found'.

3

Add error handling middleware

Add an Express error handling middleware function with four parameters: err, req, res, and next. Inside it, set the response status to 404 and send a JSON object with keys error set to true and message set to the errorMessage variable.

Express

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

app.get('/user', (req, res, next) => {
    next(new Error('User not found'));
});

const errorMessage = 'User not found';

// Your code here

Need a hint?

Use app.use with four parameters to create error middleware. Set status 404 and respond with JSON containing error: true and message: errorMessage.

4

Start the Express server

Add code to start the Express server by calling app.listen on port 3000.

Express

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

app.get('/user', (req, res, next) => {
    next(new Error('User not found'));
});

const errorMessage = 'User not found';

app.use((err, req, res, next) => {
    res.status(404).json({ error: true, message: errorMessage });
});

// Your code here

Need a hint?

Call app.listen with port 3000 to start the server.