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              class Element {
  
  constructor(listener) {
    this.listener = listener;  
  }
  
  sayHello() {
    let message = "Hello World of Elements!";
    let htmlElement = this.listener.getHtmlElement();
    htmlElement.innerHTML = message;
  }
}

class App {
  
  start() {
    // Build listener
    let elementListener = {};
    elementListener.getHtmlElement = this.getHeader;
    
    // Assemble element
    let element = new Element(elementListener);
    element.sayHello();
  }
  
  getHeader() {
    return document.getElementById("header");
  }
}

let app = new App();
app.start();

            
          
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999px
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