What does the "this." represent in the code?

MDN article on this question

If attaching a handler function to an element using addEventListener(), the value of this> inside the handler is a reference to the element. It is the same as the value of the currentTarget property of the event argument that is passed to the handler.

So yes, that line would be equivalent. It is the element that the listener is attached to.

Hamzah Iqbal , I believe in the current case that "this" refers to the actual startGameButton. So if you follow the logic here, when someone clicks the start game button, the game starts by calling

game.startGame();

then the line of code you are asking about runs

 this.style.display = 'none';
        document.getElementById('play-area').style.opacity = '1';

the "this" is pointing at the button element that the callback is created on You want to hide the button once the game has started which is exactly what it does here. It hides the start button after the game begins.

Hamzah Iqbal Yes! I'm pretty sure you could re-select the DOM element with document.querySelector("#begin-game")

I used an arrow function, which changes the scope of this, so I used event.target to get a reference to the button being pressed!

/**
 * Listens for click on `#begin-game` and calls startGame() on game object
 */
document.getElementById('begin-game').onclick = event => {
  game.startGame();

  event.target.style.display = 'none';
  document.getElementById('play-area').style.opacity = '1';
};