Environment Variables

This lesson covers Environment Variables. Think of this lesson as a short workshop you can run on your laptop.

What you will learn

• What environment variables 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: Lessons 1–9 in this course
• Previous lesson: Process Object — Complete Guide

Explain it simply

Environment variables store settings outside your code — database URLs, API keys, port numbers.

Why developers use this

• Keep secrets out of git
• Change config per machine without editing code
• Standard practice in Docker and cloud

How it works (step by step)

1. You write JavaScript in a .js file about Environment Variables.
2. You run it with node filename.js in the terminal.
3. Node prints output or starts a server depending on the lesson.
4. You change one line, run again, and see what changed — that is how you learn.

Code example — type this yourself

require('dotenv').config();
const port = process.env.PORT || 3000;
console.log('Listening on', port);

Create a .env file with PORT=4000. dotenv loads it into process.env. Never commit .env to git.

What each part does

• require('dotenv').config(); — Loads a built-in module or package you installed with npm.
• const port = process.env.PORT || 3000; — Reads config from environment variables — safe place for secrets.
• console.log('Listening on', port); — Prints to the terminal — great for learning; use proper logging in production.

Real life: where Environment Variables shows up

A startup team uses Environment Variables when they bootstrap their first API. The developer runs a small script on a laptop, stores config in .env, and splits code into modules before the app grows. Start small: one feature working beats a perfect architecture on paper.

Try it yourself — hands-on

1. npm install dotenv
2. Create .env with PORT=4000
3. Run your script and confirm the port logs

Common mistakes (avoid these)

• Pushing API keys to GitHub — rotate them immediately if that happens.

Interview note

Interviewers often ask: “What is Environment Variables?” Answer in one sentence, then give a tiny example you actually ran.

Summary

• Store secrets in .env, not in source code
• Use process.env.PORT with a default
• Add .env to .gitignore