.env.local

JWT_SECRET=dev secret

.env.production

JWT_SECRET=prod secret

.gitignore

.env.production

config.js

const dotenv = require('dotenv');

const env = process.env.NODE_ENV === 'production'
  ? '.env.production'
  : '.env.local';

dotenv.config({ path: env })

const config = {
  jwt_secret: process.env.JWT_SECRET,
}

module.exports = config;

index.js

const config = require('./config');

console.log(config);

package.json

{
  "scripts": {
    "start": "NODE_ENV=production node .",
    "dev": "NODE_ENV=development node ."
  },
  "dependencies": {
    "dotenv": "^16.3.1"
  }
}