Konami Code Lab

Objectives

1. Practice using event listeners.
2. Explain how event listeners are triggered.
3. Capture user interactions to trigger events on a page.

Instructions

In konami_code.js, you'll notice that we've provided very little: well, nothing except -- what's that? If you open the file up in your text editor, you should see:

const codes = [
  "ArrowUp",
  "ArrowUp",
  "ArrowDown",
  "ArrowDown",
  "ArrowLeft",
  "ArrowRight",
  "ArrowLeft",
  "ArrowRight",
  "b",
  "a"
];

function init() {
  // your code here
}

But what could those keycodes be? They're the famous Konami Code, as JavaScript KeyboardEvent keys. It's become a common Easter egg for sites to have hidden features behind this code, and now it's your turn to implement it!

In index.html, you'll see that we're loading the file in for you:

<script src="konami_code.js"></script>

This is JavaScript's way of pulling in code from outside the page. Here, we've given the <script> tag a local source (the file that's right here in the directory), but we could also supply a URL to load an external resource (more on that in a bit).

You'll want to attach an event listener to document.body and check for 'keydown' events. If the user enters this special code, pressing all ten of the keys in the correct order, alert() a congratulatory message. However, if they press a key out of sequence or a key that isn't part of the Konami code, don't alert() anything and simply keep listening for all ten keydowns in the correct order.

When you're testing the code out in the browser, be sure to call init() to attach the event listener and set everything up!

Stuck on how to get started? Here's a contrived, short example:

// Keys for A, B, and C keys.
const alphabet = ['a', 'b', 'c'];

// Keep track of index outside of the event handler.
let index = 0;

// This is the function that would be invoked by the event listener.
function onKeyDownHandler(e) {
  const key = e.key;

  if (key === alphabet[index]) {
    index++;

    if (index === alphabet.length) {
      alert("Hurray!");

      index = 0;
    }
  } else {
    index = 0;
  }
}

Have fun!

Hints

KeyboardEvent has gotten lots of recent updates. The key and code properties recently replaced which, keyCode, and charCode properties, which were often implemented slightly differently between different browsers and would report different values across different operating systems. Some environments (node in particular) don't know about KeyboardEvent

Resources

• Konami Code
• MDN - KeyboardEvent
• MDN — keydown

View Konami Code Lab on Learn.co and start learning to code for free.