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The process core module of Node.js provides the env property which hosts all the environment variables that were set at the moment the process was started.
process does not need to be imported — it is a global object in Node.js.

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Setting environment variables at startup

The following command runs app.js and sets USER_ID and USER_KEY: 
USER_ID=239482 USER_KEY=foobar node app.js
This passes USER_ID as 239482 and USER_KEY as foobar. This is suitable for testing; for production, you will probably be configuring bash scripts to export variables. Here is an example that accesses those environment variables: 
console.log(process.env.USER_ID); // "239482"
console.log(process.env.USER_KEY); // "foobar"
In the same way, you can access any custom environment variable you set.

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Loading variables from a .env file

Node.js 20 introduced experimental support for .env files. Use the --env-file flag to specify an environment file when running your Node.js application. 
# .env file
PORT=3000

console.log(process.env.PORT); // "3000"
Run app.js with environment variables loaded from the .env file: 
node --env-file=.env app.js
This command loads all environment variables from the .env file, making them available to the application via process.env.

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Multiple .env files

You can pass multiple --env-file arguments. Subsequent files override pre-existing variables defined in previous files: 
node --env-file=.env --env-file=.development.env app.js
If the same variable is defined in both the environment and in the file, the value from the environment takes precedence.

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Optional .env files

To avoid throwing an error if the .env file is missing, use the --env-file-if-exists flag: 
node --env-file-if-exists=.env app.js

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Loading .env files programmatically with process.loadEnvFile()

Node.js provides a built-in API to load .env files directly from your code: process.loadEnvFile(path). This method loads variables from a .env file into process.env, similar to how the --env-file flag works — but can be invoked programmatically.
Because this method is invoked post-initialization, setting startup-related environment variables (such as NODE_OPTIONS) has no effect on the process. However, these variables can still be accessed via process.env.

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Example

Given a .env file: 
PORT=1234
Load and access variables in your code: 
import { loadEnvFile } from 'node:process';

// Loads environment variables from the default .env file
loadEnvFile();

console.log(process.env.PORT); // Logs '1234'
You can also specify a custom path: 
import { loadEnvFile } from 'node:process';
loadEnvFile('./config/.env');

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