Environment variables

Node.jsJunior~2 min read

1. What environment variables are

Environment variables are key-value pairs stored by the operating system, available to any process you launch.

They exist so configuration doesn't live inside the code (especially sensitive data).

Example on Linux / macOS:

javascript

export NODE_ENV=production
export PORT=3000

Example in Windows PowerShell:

javascript

$env:NODE_ENV="development"
$env:PORT="8080"

After that, the variables are available to every process started from that session.

2. Reading environment variables in Node.js

Node.js exposes a global process.env object holding every environment variable:

javascript

console.log(process.env.NODE_ENV); // 'production'
console.log(process.env.PORT);     // '3000'

If a variable doesn't exist, it's undefined:

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console.log(process.env.API_KEY); // undefined

3. Setting variables at launch

You can set variables right in the run command.

Linux / macOS:

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NODE_ENV=production PORT=8080 node app.js

Windows (cmd.exe):

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set NODE_ENV=production && set PORT=8080 && node app.js

Windows (PowerShell):

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$env:NODE_ENV="production"; $env:PORT="8080"; node app.js

4. Using variables in code

They're usually used to configure the app:

javascript

const PORT = process.env.PORT || 3000;
const NODE_ENV = process.env.NODE_ENV || 'development';

app.listen(PORT, () => {
  console.log(`Server running on port ${PORT} in ${NODE_ENV} mode`);
});

If a variable isn't set, it's best to provide a fallback default.

5. Working with a `.env` file (dotenv)

Storing variables directly in the OS is inconvenient, especially during development. So a .env file is usually created at the project root instead.

.env

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NODE_ENV=development
PORT=4000
API_KEY=12345-ABCDE
DB_URL=postgres://user:pass@localhost/db

Installing the package:

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npm install dotenv

Loading it in code:

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import dotenv from 'dotenv';
dotenv.config();

console.log(process.env.DB_URL);

After dotenv.config() runs, every variable from .env is added to process.env.

6. Why environment variables matter

Reason	Example
Separating environments	`NODE_ENV=development` / `production`
Security	API keys, passwords, tokens don't live in the code
Flexible configuration	Behavior can change without touching the source
Portability	The same code runs on different systems (dev/staging/prod)
CI/CD integration	Variables are passed through GitHub Actions, Docker, etc.

7. A full example scenario

.env

javascript

NODE_ENV=production
PORT=8080
API_KEY=abc123

app.js

javascript

import dotenv from 'dotenv';
dotenv.config();

console.log('ENV:', process.env.NODE_ENV);
console.log('API KEY:', process.env.API_KEY);
console.log('PORT:', process.env.PORT);

if (process.env.NODE_ENV === 'production') {
  console.log('Running in production mode');
} else {
  console.log('Running in development mode');
}

Running it

javascript

node app.js

Output:

javascript

ENV: production
API KEY: abc123
PORT: 8080
Running in production mode

8. Where environment variables are used

Area	Examples
Server settings	PORT, HOST, BASE_URL
Security	JWT_SECRET, API_KEY, DB_PASSWORD
Databases	DATABASE_URL, MONGO_URI, REDIS_HOST
Integrations	AWS_ACCESS_KEY, STRIPE_SECRET_KEY
Runtime mode	NODE_ENV=development / production

9. Securing `.env`

Don't push .env to GitHub, add it to .gitignore instead:

javascript
.env
For deployment, store variables in the platform's environment settings (Heroku, Vercel, Docker, etc.)
Never log sensitive data (process.env.PASSWORD).

10. Environment variables in Docker

In a Dockerfile:

javascript

ENV NODE_ENV=production
ENV PORT=8080

or in docker-compose.yml:

javascript

environment:
  - NODE_ENV=production
  - PORT=8080

Node.js inside the container still reads them through process.env.

11. Quick summary

Point	Description
What it is	Keys and values defined in the process's environment
Access	`process.env.VARIABLE`
Setting them	Via `export` / `set` / `.env`
Library	`dotenv` for local files
Goal	Safe, flexible application configuration
Important	Never store `.env` in the repository

In one sentence:

Environment variables are a way to pass external configuration into Node.js (mode, ports, keys, passwords, and so on) through process.env, so the code stays generic, secure and independent of where it runs.

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