How to Use dotenv with Express for Environment Variables

To use dotenv with express, first install dotenv and create a .env file with your variables. Then, require and configure dotenv at the very start of your main server file before accessing any environment variables with process.env.

Syntax

Here is the basic syntax to use dotenv in an Express app:

• require('dotenv').config(); loads variables from .env into process.env.
• Access variables with process.env.VARIABLE_NAME.
• Place require('dotenv').config(); at the very top of your main file.

javascript

require('dotenv').config();

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

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

app.get('/', (req, res) => {
  res.send(`Server running on port ${port}`);
});

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

Example

This example shows a simple Express server using dotenv to load a custom port from a .env file.

The server responds with the port number it is running on.

javascript

/* .env file content (create this in your project root):
PORT=4000
*/

// index.js
require('dotenv').config();
const express = require('express');
const app = express();

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

app.get('/', (req, res) => {
  res.send(`Server running on port ${port}`);
});

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

Output

App listening on port 4000

Common Pitfalls

Common mistakes when using dotenv with Express include:

• Not calling require('dotenv').config() at the very top of the main file, so environment variables are undefined.
• Forgetting to create or properly name the .env file.
• Not restarting the server after changing the .env file.
• Committing the .env file to public repositories, exposing secrets.

javascript

/* Wrong way: dotenv config called after using env variables */

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

const port = process.env.PORT || 3000; // PORT is undefined here if dotenv not loaded

require('dotenv').config(); // Too late

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

/* Right way: dotenv config at the top */

require('dotenv').config();
const express = require('express');
const app = express();

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

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

Quick Reference

• Install dotenv: npm install dotenv
• Create .env file: Store key=value pairs, e.g., PORT=4000
• Load dotenv: require('dotenv').config(); at the top of your main file
• Use variables: Access with process.env.VARIABLE_NAME
• Security: Add .env to .gitignore to keep secrets safe

Key Takeaways

Always call require('dotenv').config() at the very start of your main Express file.

Store environment variables in a .env file and never commit it to public repos.

Access variables using process.env.VARIABLE_NAME in your Express app.

Restart your server after changing the .env file to apply updates.

Use a default value in code to avoid errors if environment variables are missing.