Environment Variables & Config

Real applications need different settings in different places — a different database URL locally versus in production, a port that might already be taken. Environment variables, accessed through process.env, are the standard way to handle that without hardcoding values into the source code.

Reading an environment variable

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

console.log(`Starting on port ${port}`);

node server.js
PORT=4000 node server.js

Starting on port 3000
Starting on port 4000

Setting PORT=4000 before the command exports it into the environment that node runs in; process.env.PORT picks it up. Without it, process.env.PORT is undefined, and || 3000 falls back to a sensible default.

Loading variables from a .env file

Typing environment variables on every command line gets old fast. Most projects instead keep them in a .env file (never committed to git) and load it with a package like dotenv:

PORT=4000
DATABASE_URL=postgres://localhost/myapp

require("dotenv").config();

console.log("Port:", process.env.PORT);
console.log("DB:", process.env.DATABASE_URL);

Port: 4000
DB: postgres://localhost/myapp

require("dotenv").config() reads the .env file and copies its values into process.env, as if you'd set them on the command line yourself.