Logging

This lesson covers Logging. You do not need to memorize everything. Understand the flow first.

What you will learn

• What logging means — in normal words, not textbook words
• How it works step by step
• Code you can run today on your laptop
• Where teams use this in real projects

Before you start

• Software: Node.js LTS from nodejs.org, VS Code, and a terminal
• Knowledge: Earlier lessons in this Node.js course
• Previous lesson: Monitoring — Complete Guide

Explain it simply

Logs record what your app did — request path, errors, user id — essential for debugging production.

Why developers use this

• Keeps apps fast and safe
• Standard in production
• Small changes, big impact

How it works (step by step)

1. Measure which endpoint or query is slow.
2. Add Logging at that bottleneck.
3. Re-test under realistic load.
4. Document what you changed for the next developer.

Code example — type this yourself

const pino = require('pino');
const log = pino();
log.info({ userId: 1 }, 'User logged in');

Use structured JSON logs in production. console.log alone is hard to search at scale.

What each part does

• const pino = require('pino'); — Loads a built-in module or package you installed with npm.
• const log = pino(); — Line 2: runs as written.
• log.info({ userId: 1 }, 'User logged in'); — Line 3: runs as written.

Real life: where Logging shows up

Before a sale event, the team applies Logging so login and product pages stay fast when traffic jumps 10× for a few hours. In interviews, explain the trade-off you chose and what you would measure in production.

Try it yourself — hands-on

1. Create a new file (e.g. logging-demo.js) in an empty folder
2. Type the example code for Logging yourself — typing helps memory
3. Run node on that file and read the output
4. Change one line (a value, a message, a route path) and run again to see what breaks or improves

Common mistakes (avoid these)

• Skipping the terminal — Logging only feels easy after you run code yourself.

Interview note

Senior interviews may ask how Logging behaves under load, failure, or security review — mention logging, timeouts, and validation.

Summary

• You can explain Logging in your own words
• You ran working code — not just read about it
• You know one mistake to avoid and one real place teams use this