How do you implement error boundaries in a Node.js application?

Question

Accepted Answer

**Error boundaries** — catch and handle errors at different levels.

```javascript
// Application-level error handler
class AppError extends Error {
    constructor(message, statusCode, code) {
        super(message);
        this.statusCode = statusCode;
        this.code = code;
    }
}

// Async handler wrapper
const asyncHandler = (fn) => (req, res, next) => {
    Promise.resolve(fn(req, res, next)).catch(next);
};

// Route with error handling
app.get("/users/:id", asyncHandler(async (req, res) => {
    const user = await User.findById(req.params.id);
    if (!user) {
        throw new AppError("User not found", 404, "USER_NOT_FOUND");
    }
    res.json({ data: user });
}));

// Global error handler
app.use((err, req, res, next) => {
    if (err instanceof AppError) {
        return res.status(err.statusCode).json({
            error: err.message,
            code: err.code,
        });
    }

console.error("Unhandled error:", err);
    res.status(500).json({ error: "Internal server error" });
});
```

**Key points:**
- Custom error classes for different types.
- Async wrapper prevents unhandled rejections.
- Global handler catches everything else.

Node.js: How do you implement error boundaries in a Node.js application?