Suggest an edit

Improve this article

Refine the answer for “Environment variables”. Your changes go to moderation before they’re published.

Approval required

Content

What you’re changing

Title (EN)

Short answer (EN)

Shown above the full answer for quick recall.

Answer (EN)

## 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`:

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

## 3. Setting variables at launch

You can set variables **right in the run command**.

### Linux / macOS:

```javascript
NODE_ENV=production PORT=8080 node app.js
```

### Windows (cmd.exe):

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

### Windows (PowerShell):

```javascript
$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

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

### Installing the package:

```javascript
npm install dotenv
```

### Loading it in code:

```javascript
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.

Markdown · drag & drop images · ⌘B / ⌘I shortcuts637 words

For the reviewer

Note to the moderator (optional)

Visible only to the moderator. Helps review go faster.