How to load a .env file in Node.js

Node.js applications often need configuration values that change between a local shell, a CI job, and a deployment target. A .env file gives a development process a repeatable set of environment variables without hard-coding those values into JavaScript source.

The built-in --env-file flag loads key-value pairs before the entry script runs, so application code reads them from process.env like any other environment variable. This startup-time path does not require the third-party dotenv package for a simple local configuration file.

Keep dotenv files trusted and local when they contain secrets. Existing shell environment values take precedence over file values, and the CLI env-file loader also parses Node startup options from NODE_OPTIONS, so a checked-in or untrusted file can change how the process starts.

Steps to load a .env file in Node.js:

Create a local .env file with development settings.
```
$ cat > .env <<'EOF'
APP_ENV=development
PORT=3000
API_TOKEN="local-dev-token"
EOF
```
Do not commit a real .env file that contains tokens, passwords, private endpoints, or account identifiers. Keep a separate .env.example with nonsecret variable names when the project needs a template.

Create app.mjs with required variable checks.

app.mjs

const required = ["APP_ENV", "PORT", "API_TOKEN"];
 
for (const name of required) {
  if (!process.env[name]) {
    console.error(`missing ${name}`);
    process.exit(1);
  }
}
 
console.log(`environment=${process.env.APP_ENV}`);
console.log(`port=${process.env.PORT}`);
console.log("apiToken=loaded");

Only the presence of API_TOKEN is printed; the token value stays out of terminal output.

Verify Node.js loads the .env file before the script starts.
```
$ node --env-file=.env app.mjs
environment=development
port=3000
apiToken=loaded
```
The env file path is resolved from the current working directory. Use a different path after --env-file= when the file is stored elsewhere.
Run the script with a command-scoped override for PORT.
```
$ PORT=4000 node --env-file=.env app.mjs
environment=development
port=4000
apiToken=loaded
```
An existing environment variable wins over the same key in the env file. The assignment syntax shown is for POSIX shells; use the equivalent syntax for the shell that starts the process.
Use the optional env-file flag when a local override file may not exist.
```
$ node --env-file=.env --env-file-if-exists=.env.local app.mjs
.env.local not found. Continuing without it.
environment=development
port=3000
apiToken=loaded
```
Place an optional local override after the base file when values in .env.local should replace matching values from .env whenever that file is present.
Remove the temporary files.
```
$ rm .env app.mjs
```