old_contents: let express = require('express');
  
  let app = express();
  
  app.use(express.static('public'));
  
  app.listen(3000, () => {
    console.log("http://localhost:3000/");
  });
  
new_contents: let express = require('express');
  
  let app = express();
  
  app.use(express.static('public'));
  
  app.listen(process.env.PORT, () => {
    console.log("http://localhost:" + process.env.PORT + "/");
  });
  
subject: Change port to use env variable.