How to Use Environment Variables in Express for Configuration

In Express, use process.env.VARIABLE_NAME to access environment variables. Load them from a .env file using the dotenv package for easy configuration management.

Syntax

Environment variables in Express are accessed via process.env. You can read any variable by its name, for example, process.env.PORT to get the port number.

To load variables from a file, use the dotenv package and call require('dotenv').config() at the start of your app.

javascript

require('dotenv').config();

const port = process.env.PORT || 3000;

console.log(`Server will run on port: ${port}`);

Output

Server will run on port: 3000

Example

This example shows how to create a simple Express server that uses environment variables for the port and a secret key. It loads variables from a .env file using dotenv.

javascript

import express from 'express';
import dotenv from 'dotenv';

dotenv.config();

const app = express();
const port = process.env.PORT || 3000;
const secretKey = process.env.SECRET_KEY || 'defaultsecret';

app.get('/', (req, res) => {
  res.send(`Secret key is: ${secretKey}`);
});

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

Output

Server running on port 4000

Common Pitfalls

• Forgetting to call dotenv.config() before accessing process.env causes variables to be undefined.
• Not creating a .env file or not placing it in the project root leads to missing variables.
• Using strings like process.env.PORT directly without fallback can crash your app if the variable is missing.
• Committing .env files with secrets to public repos risks security leaks.

javascript

/* Wrong way: Missing dotenv config */
console.log(process.env.API_KEY); // undefined

/* Right way: */
require('dotenv').config();
console.log(process.env.API_KEY); // prints the key if set

Quick Reference

• Install dotenv: npm install dotenv
• Create a .env file in your project root
• Put variables like PORT=4000 and SECRET_KEY=mysecret in .env
• Load variables early with require('dotenv').config()
• Access variables with process.env.VARIABLE_NAME

Key Takeaways

Always load environment variables early using dotenv.config() in Express apps.

Access variables safely with process.env.VARIABLE_NAME and provide defaults.

Never commit .env files with secrets to public repositories.

Use environment variables to keep configuration flexible and secure.

Create a .env file in your project root to store variables for local development.