Lesson 9: Error Handling in Express

🎯 Learning Objectives

By the end of this lesson, you will be able to:

• Implement comprehensive error handling
• Create custom error classes
• Use error handling middleware
• Handle different types of errors
• Implement logging and monitoring
• Create user-friendly error responses

📚 Error Handling Patterns

Custom Error Classes

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

class ValidationError extends AppError {
    constructor(message) {
        super(message, 400);
    }
}

Error Handling Middleware

function errorHandler(err, req, res, next) {
    let error = { ...err };
    error.message = err.message;

    // Mongoose bad ObjectId
    if (err.name === 'CastError') {
        const message = 'Resource not found';
        error = new AppError(message, 404);
    }

    // Mongoose duplicate key
    if (err.code === 11000) {
        const message = 'Duplicate field value entered';
        error = new AppError(message, 400);
    }

    // Mongoose validation error
    if (err.name === 'ValidationError') {
        const message = Object.values(err.errors).map(val => val.message);
        error = new AppError(message, 400);
    }

    res.status(error.statusCode || 500).json({
        success: false,
        error: error.message || 'Server Error'
    });
}

🎯 Practice Exercises

Exercise 1: Complete Error Handling System

Create a comprehensive error handling system:

Solution:

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

const app = express();
app.use(express.json());

// Custom Error Classes
class AppError extends Error {
    constructor(message, statusCode) {
        super(message);
        this.statusCode = statusCode;
        this.isOperational = true;
        Error.captureStackTrace(this, this.constructor);
    }
}

class ValidationError extends AppError {
    constructor(message) {
        super(message, 400);
    }
}

class NotFoundError extends AppError {
    constructor(resource) {
        super(`${resource} not found`, 404);
    }
}

// Error Handling Middleware
function errorHandler(err, req, res, next) {
    let error = { ...err };
    error.message = err.message;

    // Log error
    console.error(err);

    // Mongoose bad ObjectId
    if (err.name === 'CastError') {
        const message = 'Resource not found';
        error = new AppError(message, 404);
    }

    // Mongoose duplicate key
    if (err.code === 11000) {
        const message = 'Duplicate field value entered';
        error = new AppError(message, 400);
    }

    // Mongoose validation error
    if (err.name === 'ValidationError') {
        const message = Object.values(err.errors).map(val => val.message);
        error = new AppError(message, 400);
    }

    // JWT errors
    if (err.name === 'JsonWebTokenError') {
        const message = 'Invalid token';
        error = new AppError(message, 401);
    }

    if (err.name === 'TokenExpiredError') {
        const message = 'Token expired';
        error = new AppError(message, 401);
    }

    res.status(error.statusCode || 500).json({
        success: false,
        error: error.message || 'Server Error',
        ...(process.env.NODE_ENV === 'development' && { stack: err.stack })
    });
}

// Async Error Handler
function asyncHandler(fn) {
    return (req, res, next) => {
        Promise.resolve(fn(req, res, next)).catch(next);
    };
}

// Example route with error handling
app.get('/api/users/:id', asyncHandler(async (req, res, next) => {
    const user = await User.findById(req.params.id);

    if (!user) {
        return next(new NotFoundError('User'));
    }

    res.json({
        success: true,
        data: user
    });
}));

// Global error handler (must be last)
app.use(errorHandler);

app.listen(3000, () => {
    console.log('Server running on http://localhost:3000');
});

🎯 Key Takeaways

1. Custom error classes provide specific error types
2. Error middleware handles all errors centrally
3. Async error handling catches promise rejections
4. Error logging helps with debugging
5. User-friendly messages improve experience

Next Lesson: Lesson 10: Mini Project - Simple Blog API