01 Dec 2019

This article will take about 4 minutes to read.
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I love easter eggs in software.

You may have noticed the keyboard image in the background of my blog. This thing on the right side of the screen. Those with a keen eye figured out it highlights keys as you are typing. It has been here for some time now, and on my colleague’s request, I’ll explain how it works.

Let’s break it down.

Vector image

First I needed a vector (SVG) keyboard image. SVG images are easy to manipulate using JavaScript. I ended making one myself, by drawing over an image of the mac keyboard.

To target keys easily I added id to every key, and it looks something like this:

<rect class="Key" id="Key--q" x="52" y="70" width="36" height="37" rx="6" />
<rect class="Key" id="Key--w" x="95" y="70" width="36" height="37" rx="6" />
<rect class="Key" id="Key--e" x="138" y="70" width="36" height="37" rx="6" />
<rect class="Key" id="Key--r" x="182" y="70" width="36" height="37" rx="6" />
<rect class="Key" id="Key--t" x="225" y="70" width="36" height="37" rx="6" />
<rect class="Key" id="Key--y" x="269" y="70" width="36" height="37" rx="6" />

Key codes

You probably already know this, but KeyboardEvent has I just learned keyCode is deprecated in favor of code. It is much better as it is not altered by keyboard layout or the state of the modifier keys. But it is not supported in Edge and IE, so I’ll stick to keyCode in this post. We are going to use this property to map physical key presses to keys in our image.

I found a detailed list of key codes, and created a large mapper based on it. Object keys are keyCode values and each one is mapped to the html id I set. For keys that exist on both side of the keyboard (shift, ctrl, cmd…) I added checkSide property. This property tells us to check which of the two is pressed, so we can highlight the correct one.

const keyCodes = {
  8: {
    id: 'delete',
  },
  9: {
    id: 'tab',
  },
  13: {
    id: 'enter',
  },
  16: {
    id: 'shift',
    checkSide: true,
  },
  // ...
  // all the way to key code 255
};

Logic

On both keydown and keyup we need to find the corresponding element on our vector keyboard, and add or remove the active class.

document.addEventListener('keydown', function(e) {
  const keyElement = getKeyElement(e);

  // If element is found add the active class
  if (keyElement) {
    // I'm using setAttribute because IE doesn't support classList on SVG elements
    keyElement.setAttribute('class', 'Key Key--active');
  }
});

document.addEventListener('keyup', function(e) {
  const keyElement = getKeyElement(e);

  // If element is found remove the active class
  if (keyElement) {
    // I'm using setAttribute because IE doesn't support classList on SVG elements
    keyElement.setAttribute('class', 'Key');
  }
});

This is the function used for both events. It queries for the element based on the mapper and html id. It also takes in consideration checkSide flag to make sure key on the correct side is returned.

function getKeyElement(e) {
  const key = keyCodes[e.keyCode];

  if (key && key.id) {
    let keySelector = `#Key--${ key.id }`;

    // Check which key is pressed if key exists on the both sides
    if (key.checkSide && KeyboardEvent) {
      if (e.location === KeyboardEvent.DOM_KEY_LOCATION_LEFT) {
        keySelector += 'Left';
      } else if (e.location === KeyboardEvent.DOM_KEY_LOCATION_RIGHT) {
        keySelector += 'Right';
      }
    }

    const keyElement = document.querySelector(keySelector);

    return keyElement;
  }
}

When window loses focus, keys would stay highlighted. To make sure that doesn’t happen, on window blur we are going to remove all active classes from our keys.

function removeActiveKeyClass() {
  const keyElements = document.querySelectorAll('.Key--active');

  for (let i = 0; i < keyElements.length; i++) {
    keyElements[i].setAttribute('class', 'Key');
  }
}

window.addEventListener('blur', removeActiveKeyClass);

Actual code used on this page is identical and you can find it here.

Conclusion

When I initially decided to add an easter egg to my website, I was thinking of using Konami Code or even creating a tiny video game. But in the end, I really loved the keyboard one, as it was easy to find and fun to play with.

And for the end, to see how everything fits together, I created a demo for you to play with:

Category
Music

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