Debugging and logging

Last reviewed May 28, 2026 Content v20260528

Track mode: nodejs_server
Means: Node sandbox
Reading: ~1 min
Level: intermediate

This lesson

This lesson teaches Debugging and logging: the syntax, APIs, and habits you need before advancing in Node.js.

Teams ship Debugging and logging on every Node.js codebase—skipping it leaves gaps in debugging and code reviews.

You will apply Debugging and logging in contexts like: REST/GraphQL APIs, BFF layers, CLIs, webhooks, and real-time services (with WebSockets).

Run JavaScript on the Node runner when configured—never mix arbitrary shell commands in lessons.

When you can explain the previous lesson's ideas without copying starter code.

Production Node apps need structured logs and reproducible debugging—not scattered console.log. Learn levels, correlation IDs, and the built-in debugger.

Logging levels

error — failures requiring attention
warn — degraded but running
info — request milestones, startup
debug — verbose dev-only detail

Structured JSON logs

logger.info({ reqId, method, url, ms }, 'request completed');

Tools like Datadog, CloudWatch, and Loki parse JSON fields for search and alerts.

Debugging

node --inspect — Chrome DevTools debugger
VS Code launch.json — breakpoints in handlers
Reproduce with fixed NODE_ENV and env vars

Important interview questions and answers

Q: console.log in production?
A: Acceptable for small apps; structured loggers (pino) add levels, redaction, and performance.
Q: Correlation ID?
A: Unique per request propagated through logs—trace one user action across microservices.

Self-check

Why log as JSON in production?
What flag enables Node inspector?

Interview tip Lesson completion confidence

Can you explain this lesson in 30 seconds without reading notes?

Self-reflection (saved on this device)

Not saved yet.

Playground

Runs on the configured server runner (dev: npm run runner with LEARNING_RUNNER_ENABLED=true). Output appears below the editor.

Code runner not available

Server runner is disabled. Set LEARNING_RUNNER_ENABLED=true and LEARNING_RUNNER_URL in .env (see .env.example).

Check yourself

Multiple choice — immediate feedback.

Discussion

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Starter discussion topics

Structured logs why?
DEBUG env pattern?

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