How do you implement a custom error logging service in Node.js?

Question

Accepted Answer

**Error logging** — centralized error tracking and alerting.

```javascript
class ErrorLogger {
    constructor(config) {
        this.config = config;
        this.logger = pino({ level: "error" });
    }

async log(error, context = {}) {
        const entry = {
            timestamp: new Date().toISOString(),
            level: "error",
            message: error.message,
            stack: error.stack,
            code: error.code,
            ...context,
        };

// Log locally
        this.logger.error(entry);

// Send to external service
        if (this.config.sentryDsn) {
            Sentry.captureException(error, {
                extra: context,
            });
        }

// Alert on critical errors
        if (error.severity === "critical") {
            await this.alert(error, context);
        }
    }

async alert(error, context) {
        await fetch(this.config.alertWebhook, {
            method: "POST",
            headers: { "Content-Type": "application/json" },
            body: JSON.stringify({
                text: `Critical error: ${error.message}`,
                context,
            }),
        });
    }
}

// Express error handler
app.use((err, req, res, next) => {
    errorLogger.log(err, {
        url: req.url,
        method: req.method,
        userId: req.user?.id,
        requestId: req.id,
    });
    res.status(err.status || 500).json({ error: "Internal error" });
});
```

Node.js: How do you implement a custom error logging service in Node.js?