Environment Variables

Environment variables hold configuration outside your code, such as ports, API keys, and database URLs. Node.js exposes them through process.env.

Reading a variable

console.log(process.env.HOME);  // the user's home folder
console.log(process.env.PORT);  // undefined if not set

Every value in process.env is a string.

Setting a variable when running

You can set a variable just for one run:

// In the terminal:
// PORT=4000 node app.js

// In app.js:
const port = process.env.PORT || 3000;
console.log("Listening on port", port);

The || 3000 provides a default when the variable is missing.

Why use them

• Keep secrets out of your source code.
• Change behavior between development and production.
• Avoid hardcoding values that differ per machine.

Loading from a file

Many projects keep variables in a .env file and load them with a package like dotenv. Never commit secrets to version control.

Summary

Use process.env to read configuration at runtime. Provide sensible defaults and keep sensitive values out of your code.