Use the .env File

In ‘basic node and express’ section I got stuck in the section of ‘use .env file’.
My code is:

let express = require('express');
let app = express();

console.log("Hello World");


app.use('/public', express.static(__dirname + '/public'))

app.get("/",(req, res)=>{
  const filePath = __dirname + '/views/index.html';
  res.sendFile(filePath);
});

app.get('/json',function(req,res){
  const helloJSON = "Hello json";
  let message;
  if(process.env.MESSAGE_STYLE === 'uppercase'){
    message = helloJSON.toUpperCase();
  }else{
    message=helloJSON;
  }
  const data={
    "message":message;
  };
  res.json(data);
})



































 module.exports = app;

Do you have a link to your project’s full code?