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, just about nothing except -- what's that? If you open the file up in your text editor, you should see:

const code = [38, 38, 40, 40, 37, 39, 37, 39, 66, 65]

function init() {
  // your code here
}

But what could those numbers be? They're the famous Konami Code, as keyboard event values. 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:

// Key codes for A, B, and C keys.
const alphabet = [65, 66, 67];

// 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 = parseInt(e.detail || e.which);

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

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

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

Have fun!

Hints

Be aware that we're looking at both e.detail and e.which. In modern browsers, e.which is usually sufficient; however, it's not universally supported, it's technically deprecated, and it sometimes exhibits finicky behavior in the test suite. To be safe, you might want to check e.detail or e.location. Or, to be safest, read the latest in the keydown reference on MDN. This would be a good chance to use console.log() to check out the value of e.detail, e.which, and e.location.

Also note that we're calling parseInt on the key value. This is because the event handler might pass us the string representation of the number, which wouldn't work so well with our comparisons.

Resources

• Konami Code
• MDN — keydown

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