Express.js Error Handling

Error handling is crucial for building robust applications. In Express.js, error handling middleware helps catch and manage errors gracefully, ensuring better debugging and user experience.

 

Key Features of Error Handling

• Centralized Error Management: Handle errors in one place.
• Custom Error Responses: Provide user-friendly error messages.
• Debugging Support: Log errors for troubleshooting.
• Integration with Middleware: Seamlessly integrate with existing middleware stacks.

 

Setting Up Error Handling in Express.js

Default Error Handler
Express.js provides a built-in error handler that sends error stack traces in development mode.

Example:

app.use((err, req, res, next) => {
    console.error(err.stack);
    res.status(500).send('Something went wrong!');
});

Custom Error Middleware
Create custom middleware to handle specific error scenarios.

Example:

app.use((err, req, res, next) => {
    if (err.type === 'custom') {
        res.status(400).send(err.message);
    } else {
        next(err);
    }
});

Handling 404 Errors
Define middleware to handle requests to undefined routes.

Example:

app.use((req, res, next) => {
    res.status(404).send('Page not found!');
});

Error Throwing in Routes
Use next(err) to forward errors to error-handling middleware.

Example:

app.get('/example', (req, res, next) => {
    try {
        throw new Error('Example error');
    } catch (err) {
        next(err);
    }
});

Using Asynchronous Error Handlers
Handle errors in asynchronous functions using try-catch.

Example:

app.get('/async', async (req, res, next) => {
    try {
        const data = await someAsyncFunction();
        res.send(data);
    } catch (err) {
        next(err);
    }
});

Error Logging
Log errors using libraries like winston or morgan.

Example:

const morgan = require('morgan');
app.use(morgan('dev'));

 

Complete Example

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

// Middleware for 404 errors
app.use((req, res, next) => {
    res.status(404).send('Resource not found');
});

// Error handling middleware
app.use((err, req, res, next) => {
    console.error(err.stack);
    res.status(500).send('Internal Server Error');
});

// Example route
app.get('/error', (req, res, next) => {
    next(new Error('Sample error!'));
});

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

 

Summary

Express.js error handling provides a structured approach to managing errors in applications. By implementing error-handling middleware, developers can create user-friendly responses, streamline debugging, and ensure application reliability.