Advent of JavaScript, Day 3

Advent of JS Homepage

Challenge #3 is creating a keyboard game:

Screenshot of keyboard

With the project files downloaded and codesandbox‘d into a live CodeSandbox, I’m ready to get going!

Again, let’s start with the User Requirements and speculate how I can solve these:

Users should be able to:

  • See the keyboard centered on the page


  • Whenever a user hovers over a specific key it will change colors
    • White keys will change to yellow #ffd200


  • Black keys will change to pink #f40082


  • When a user clicks on a specific key, it will play an audio clip.

This seems straightforward – new Audio(url).

  • The audio clips are numbered, but I did not specifically number the keys. You can pick which key should be associated with each audio file.

The files are named audio/key-{1-23}.mp3, so the initial implementation can just increment by key position.

  • If a user clicks on one key, then immediately clicks on a second key. The 2 files should both play. Meaning, clicking on one key will not stop an existing audio file from playing.

What this tells me is that there won’t be a re-use of variable references.

Initial Approach

  • Vanilla JS (no libraries), since there doesn’t seem to be much complexity here

  • document.querySelectorAll('.piano a') to find all keys

  • .addEventListener('click', ...) to listen for clicks

  • new Audio(url) to create & .play the audio

    The URLs load asynchronously, so it may be best to initialize these immediately and .play once clicked.

Making it Work

This wasn’t much of a challenge. I was able to get this completed while an episode of Spongebob was playing…

Anyway, the only surprise was with the SVG keys being out of order: the <a> elements aren’t in left-to-write order, but seemingly random in the SVG.

To solve this, I used .sort to find the mid-point of each key and order those left-to-right:

const keyElements = Array.from(document.querySelectorAll('.piano a'))

keyElements.sort((a, b) => {
  const aRect = a.getBoundingClientRect()
  const aMiddle = aRect.left + aRect.width / 2

  const bRect = b.getBoundingClientRect()
  const bMiddle = bRect.left + bRect.width / 2

  return aMiddle - bMiddle

Then, I mapped those keys to the sound they would play:

const keyAudios =, i) => {
  return new Audio(`audio/key-${i + 1}.mp3`)

(The browser automatically marks these as preload, so they should be ready to go once you click a key.)

One way to associate clicks-to-keys is to keyElements.forEach(keyElement, ...) and keyElement.addEventListener('click', ...).

An alternative is to use window.addEventListener('click', ...) so there’s one event listener rather than N (23).

This optimization isn’t necessary here, but can be a good idea when indescriminately adding events to an unknown number of elements:

  • Points on a graph
  • Rows in a table
  • Links on a page
  • etc.
window.addEventListener('click', (event) => {
  // click is on the `<path>`, so traverse up to the <a>
  const a =

  if (!keyElements.includes(a)) {


  const i = keyElements.indexOf(a)
  const audio = keyAudios[i]

  audio.currentTime = 0

Since each keyAudios is separate, the sounds automatically overlap.

But, I wanted to make sure repeatedly clicking a key would repeat the sound from the beginning. The Audio API does not have a .stop or .restart method, so setting audio.currentTime = 0 effectively accomplishes the same thing.