💡 Understanding Error Handling in Express.js

Error handling is an essential part of building stable and secure applications. Express.js provides a special type of middleware that catches and manages errors in a centralized place.

🔍 Two Types of Errors

• Operational Errors — predictable issues that may occur in real-world usage.
• Programming Errors — bugs introduced by developers in code.

Operational errors are predictable issues that will happen at some point in the future. These errors are known as runtime errors that you can handle in advance.

📝 Examples:

• Accessing an invalid route
• Submitting invalid data
• Server connection failure
• Request timeout

Programming errors are mistakes made by developers. These are bugs inside the code and are usually not predictable.

📝 Examples:

• Trying to access a property of undefined
• Using await without async
• Passing wrong data types to functions

Express.js uses a special middleware with four parameters to catch errors: (err, req, res, next).

This function is used to log, format, and send error responses from one place.

function errorHandler(err, req, res, next) {
    // Handle the error
}
        

Complete working example demonstrating routes, 404 page, and error handling.

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

// Normal routes
app.get('/', (req, res) => {
    res.send('Home Page');
});

app.get('/about', (req, res) => {
    // Intentional mistake to trigger error
    res.sen('About Page'); // Wrong method name
});

// 404 Middleware - Handles undefined routes
app.use((req, res) => {
    res.status(404).send('<h1>404 Not Found</h1>');
});

// Error-handling middleware (must have 4 parameters)
app.use((err, req, res, next) => {
    console.error(err.stack);
    res.status(500).send('Something broke!');
});

// Start server
app.listen(8080, () => {
    console.log('Server is running at http://localhost:8080');
});

Express provides these built-in middlewares:

• express.json() — Parse incoming JSON data
• express.urlencoded() — Parse form-data
• express.static() — Serve static files

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

// Built-in middleware examples in ExpressJS

app.use(express.json());
app.use(express.urlencoded({ extended: true }));
app.use(express.static('public')); // Static folder

app.listen(8080, () => {
    console.log("Server running at http://localhost:8080");
});

Useful third-party middleware from npm:

Middleware	Use
cookie-parser	Parse cookies
helmet	Security headers
morgan	HTTP request logger
body-parser	Parse request bodies
cors	Enable CORS
multer	Handle file uploads
express-session	Session handling
compression	Gzip response compression

Type	Description
Application-Level	Added using app.use()
Router-Level	Applied to specific router objects
Error-Handling	Middleware with 4 parameters
Built-in	Provided by Express (json, static)
Third-Party	External npm packages