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JavaScript Object-Oriented JavaScript: Challenge Rendering the Game Connecting with the DOM

Tim Oltman
Tim Oltman
7,730 Points

When using an arrow function 'this' doesn't work inside the callback. Why?

When I set up the callback function as an arrow function I need to provide an event parameter and use event.target.style.display = 'none'. However, in the solution, they are able to use the 'this' keyword by defining the callback function as an anonymous function with no parameters. I'm just curious why it works this way.

To be specific, the following code results in: Uncaught TypeError: Cannot set property of 'display'.

document.getElementById('begin-game').addEventListener('click', () => {
    game.startGame();
    this.style.display = 'none';
    document.getElementById('play-area').style.opacity = '1';
})

However, only changing the function definition allows the code to work.

document.getElementById('begin-game').addEventListener('click', function() {
    game.startGame();
    this.style.display = 'none';
    document.getElementById('play-area').style.opacity = '1';
})
Rich Donnellan
Rich Donnellan
Treehouse Moderator 27,696 Points

Question updated with code formatting. Check out the Markdown Cheatsheet below the Add an Answer submission for syntax examples.

3 Answers

Rich Donnellan
MOD
Rich Donnellan
Treehouse Moderator 27,696 Points

Arrow functions don't have their own bindings to this, arguments, super, or new.target keywords. Your second example is exactly what it takes if you do require any of these.

This article might help explain things better: https://www.codementor.io/@dariogarciamoya/understanding-this-in-javascript-with-arrow-functions-gcpjwfyuc

Bobby Verlaan
Bobby Verlaan
29,538 Points

Please mind you can still use an arrow function. Instead of using this, you can use the event object and select the button that was clicked on like this:

const btnStartGame = document.querySelector('button#begin-game');
btnStartGame.addEventListener('click', (e) => {
  game.startGame();
  e.target.style.display = 'none';
  document.getElementById('play-area').style.opacity = '1';
});
Tim Oltman
Tim Oltman
7,730 Points

Thanks for the reply. I thought that an arrow function and an anonymous function were completely equivalent.