learnjavascript

I have been reading this book "Modern Javascript for the Impatient", chapter "Object-Oriented Programming". At the end there is an "Exercise" section. This is a question from it I need help answering:

Harry tries this code to toggle a CSS class when a button is clicked:

const button = document.getElementById('button1')
button.addEventListener('click', function () {
  this.classList.toggle('clicked')
})

It doesn’t work. Why?

Sally, after searching the wisdom of the Internet, suggests:

button.addEventListener('click', event => {
  event.target.classList.toggle('clicked')
})

This works, but Harry feels it is cheating a bit. What if the listener hadn’t produced the button as event.target? Fix the code so that you use neither this nor the event parameter.

I have been reading a book on JS. This is one of the questions at the end of a chapter that stumped me; don't know where to begin. So please help me out:

Question

A classic example for an abstract class is a tree node. There are two kinds of nodes: those with children (parents) and those without (leaves).

class Node {
  depth() { throw Error("abstract method") }
}
class Parent extends Node {
  constructor(value, children) { . . . }
  depth() { return 1 + Math.max(...children.map(n => n.depth())) }
}
class Leaf extends Node {
  constructor(value) { . . . }
  depth() { return 1 }
}

This is how you would model tree nodes in Java or C++. But in JavaScript, you don’t need an abstract class to be able to invoke n.depth().

Rewrite the classes without inheritance and provide a test program.

So, how to write this code without using inheritance in JS?