Custom error classes in Express - Mini Project: Build & Apply

Custom Error Classes in Express

📖 Scenario: You are building a simple Express server that handles user requests. You want to create custom error classes to manage different error types clearly and send proper responses.

🎯 Goal: Create custom error classes for NotFoundError and ValidationError, then use them in an Express route to handle errors properly.

📋 What You'll Learn

Create a base error class called AppError that extends Error

Create a NotFoundError class that extends AppError with a default message

Create a ValidationError class that extends AppError with a default message

Use these custom errors in an Express route and handle them in error middleware

💡 Why This Matters

🌍 Real World

Custom error classes help organize error handling in web servers, making it easier to send clear messages to users and debug issues.

💼 Career

Understanding custom errors and middleware is essential for backend developers working with Express or similar frameworks to build robust APIs.

1

Create the base error class

Create a class called AppError that extends the built-in Error class. It should accept message and statusCode parameters in the constructor and set them as properties.

Express

# Your code here
// Create class AppError extending Error with constructor(message, statusCode)

Need a hint?

Remember to call super(message) inside the constructor to set the error message.

2

Create NotFoundError and ValidationError classes

Create two classes: NotFoundError and ValidationError. Both should extend AppError. The NotFoundError constructor should call super with message 'Resource not found' and status code 404. The ValidationError constructor should call super with message 'Invalid input' and status code 400.

Express

class AppError extends Error {
    constructor(message, statusCode) {
        super(message);
        this.statusCode = statusCode;
        this.name = this.constructor.name;
        Error.captureStackTrace(this, this.constructor);
    }
}

// Your code here
// Create NotFoundError and ValidationError classes extending AppError

Need a hint?

Use super in each constructor to pass the message and status code to AppError.

3

Use custom errors in an Express route

Create an Express app. Add a GET route at /items/:id. Inside the route, if req.params.id is not '1', throw a NotFoundError. Otherwise, send JSON { id: '1', name: 'Item One' }.

Express

class AppError extends Error {
    constructor(message, statusCode) {
        super(message);
        this.statusCode = statusCode;
        this.name = this.constructor.name;
        Error.captureStackTrace(this, this.constructor);
    }
}

class NotFoundError extends AppError {
    constructor() {
        super('Resource not found', 404);
    }
}

class ValidationError extends AppError {
    constructor() {
        super('Invalid input', 400);
    }
}

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

// Your code here
// Add GET /items/:id route that throws NotFoundError if id !== '1'

Need a hint?

Use next() to pass the error to Express error handlers.

4

Add error handling middleware

Add an Express error-handling middleware function that takes err, req, res, next. If err is an instance of AppError, respond with err.statusCode and JSON { error: err.message }. Otherwise, respond with status 500 and JSON { error: 'Internal Server Error' }. Export the app.

Express

class AppError extends Error {
    constructor(message, statusCode) {
        super(message);
        this.statusCode = statusCode;
        this.name = this.constructor.name;
        Error.captureStackTrace(this, this.constructor);
    }
}

class NotFoundError extends AppError {
    constructor() {
        super('Resource not found', 404);
    }
}

class ValidationError extends AppError {
    constructor() {
        super('Invalid input', 400);
    }
}

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

app.get('/items/:id', (req, res, next) => {
    if (req.params.id !== '1') {
        return next(new NotFoundError());
    }
    res.json({ id: '1', name: 'Item One' });
});

// Your code here
// Add error handling middleware and export app

Need a hint?

Error middleware in Express has four parameters. Use instanceof to check error type.