old_contents: 'use strict';
  
  let {Main} = require('./Main');
  let component = new Main({});
  document.getElementById('main').appendChild(component.element);
  
new_contents: 'use strict';
  
  const {Main} = require('./Main');
  const component = new Main({});
  document.getElementById('main').appendChild(component.element);
  
subject: use const for Main and component