Logging for Frontend and Node.js

Use these notes to explain how you design low-overhead logging pipelines for client and server environments.

Principles

• Structured logs: Prefer JSON with consistent keys.
• Contextual logging: Include requestId, userId, and feature flags.
• Redaction: Remove PII and secrets before shipping logs.

Frontend logging

• Log UI errors via window.onerror and unhandledrejection.
• Sample high-volume logs to control cost.
• Batch logs before sending to a backend collector.

window.addEventListener('error', (event) => {
  logger.error('ui_error', { message: event.message, stack: event.error?.stack });
});

Node.js logging

• Use middleware to attach request context.
• Log at boundaries (incoming request, upstream call, DB call).
• Keep logs consistent across services.

app.use((req, _res, next) => {
  req.id = crypto.randomUUID();
  next();
});

Log levels

• debug: Dev-only details.
• info: Normal operations.
• warn: Unexpected but recoverable.
• error: Failures that need attention.

Interview prompts

• How do you correlate client logs with server traces?
• When should you sample logs?
• How do you prevent logging sensitive data?