Error Handling
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Author & Reviewer
Learn how to gracefully handle crashes, bad requests, and unexpected errors in your Express applications using Global Error Middleware.
Learning Goals
Video Tutorial Coming Soon
Our expert team is currently recording the visual guide for Error Handling.
The Core Concept
If a user requests a file that doesn't exist, or a database goes offline, your Node.js application will encounter an error. If you don't handle that error, your entire server will crash, kicking everyone offline.
In Express, the best way to handle errors is to use a **Global Error Handler**. This is a special middleware function placed at the very end of your app. Whenever something goes wrong, you pass the error to this middleware, which sends a clean JSON error message to the user instead of leaking confusing stack traces.
### Best Practices
- **Use try/catch:** Always wrap `async/await` database calls in `try/catch` blocks.
- **Don't leak secrets:** Never send raw database error messages to the user. Send generic messages like 'Internal Server Error'.
### Common Mistakes
- **Unhandled Promise Rejections:** Forgetting to catch an error in an async function will crash modern Node.js applications.
### Interview Questions 1. **How do you define an error-handling middleware in Express?** *Answer:* By defining a middleware function with exactly four arguments: (err, req, res, next).
Visual guide
Node.js concept flow
A simple original diagram to connect the lesson idea with real project flow.
Code & Implementation
const express = require('express');
const app = express();
// 1. A route that simulates a database failure
app.get('/api/data', async (req, res, next) => {
try {
// Simulating a crash
throw new Error("Database connection timed out!");
} catch (error) {
// Pass the error to the global handler
next(error);
}
});
// 2. Global Error Handler Middleware (MUST have 4 arguments)
app.use((err, req, res, next) => {
console.error("LOGGED ERROR:", err.message);
// Send a clean, professional response to the user
res.status(500).json({
success: false,
message: "Something went wrong on our end. Please try again later."
});
});
app.listen(3000, () => console.log('Server running with Error Handling!'));Expected Output
Server running with Error Handling!
(Visiting /api/data in browser shows:)
{
"success": false,
"message": "Something went wrong on our end. Please try again later."
}Custom 404 Route handler
Hands-on practice task
The Challenge
In addition to server errors, you need to handle 'Page Not Found' errors. Write an Express middleware placed after your routes (but before your error handler) that catches all unmatched routes and sends a 404 JSON response.
Helpful Hints
- •Use app.use((req, res, next) => { ... }) without a specific path.
- •Set res.status(404) and send a custom JSON message.
Quick Knowledge Check
What happens if I don't catch an error in Express?
Continue Learning
Next steps after this lesson
In addition to server errors, you need to handle 'Page Not Found' errors. Write an Express middleware placed after your routes (but before your error handler) that catches all unmatched routes and sends a 404 JSON response.
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This curriculum was designed by senior software engineers to simulate real-world production environments. We focus on practical, project-based learning instead of abstract theory.
- ✓ Content Review Date: 7/9/2026
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