Interface.js

class App {
  constructor(root) {
    this.interface = new Interface(root);
  }
}

class Interface {
  constructor(root) {
    root.innerHTML = '<button id="testButton">click me!</button>';

    this.root = root;
    this.testButton = this.root.querySelector('#testButton');

    this.testButton.onclick = this.testButtonClick;
  }

  testButtonClick(e) {
    console.log('clicked!', e);
  }
}

new App(document.querySelector('#app'));