Why process.env Is Undefined in Node.js (And How to Fix It)

# Install dotenv package
npm install dotenv

# Or with yarn
yarn add dotenv

// server.js - MUST be at the very top
require('dotenv').config();

const express = require('express');
const app = express();

// Now process.env is available
const PORT = process.env.PORT || 3000;
const DB_URL = process.env.DATABASE_URL;

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

# .env file (in project root)
PORT=3000
DATABASE_URL=mongodb://localhost:27017/mydb
API_KEY=your-api-key-here
NODE_ENV=development

# No quotes needed for values
# No spaces around = sign
# Comments start with #

# .gitignore
node_modules/
.env
.env.local
.env.*.local
dist/
build/

require('dotenv').config();

// Access with defaults
const PORT = process.env.PORT || 3000;
const NODE_ENV = process.env.NODE_ENV || 'development';

// Convert to number if needed
const PORT_NUM = parseInt(process.env.PORT) || 3000;

// Access nested or complex values
const DB_CONFIG = {
  url: process.env.DATABASE_URL,
  name: process.env.DB_NAME || 'mydb'
};

// Check if variable exists
if (!process.env.API_KEY) {
  throw new Error('API_KEY is required');
}

# Set environment variables
heroku config:set PORT=3000
heroku config:set DATABASE_URL=mongodb://...

# View all variables
heroku config

# Remove variable
heroku config:unset PORT

# Via CLI
vercel env add PORT

# Or via Vercel Dashboard
# Settings > Environment Variables
# Add: PORT = 3000

# docker-compose.yml
services:
  app:
    environment:
      - PORT=3000
      - DATABASE_URL=mongodb://...

# Or via command line
docker run -e PORT=3000 -e DATABASE_URL=... myapp