A Custom Element radio group which demonstrates roving tabindex

radio-group.html

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Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
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limitations under the License.
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<style>
  .demo {
    margin-left: 80px;
  }

  radio-button {
    position: relative;
    display: block;
    font-size: 18px;
  }

  radio-button:focus {
    outline: none;
  }

  radio-button::before {
    content: '';
    display: block;
    width: 10px;
    height: 10px;
    border: 1px solid black;
    position: absolute;
    left: -18px;
    top: 7px;
    border-radius: 50%;
  }

  radio-button:focus::before {
    box-shadow: 0 0 3px 3px #83BEFF;
  }

  radio-button[aria-checked="true"]::before {
    content: '';
    display: block;
    width: 10px;
    height: 10px;
    background: red;
    position: absolute;
    left: -18px;
    top: 7px;
    border-radius: 50%;
  }
</style>

<div class="demo">
  <radio-group>
    <radio-button>Water</radio-button>
    <radio-button>Coffee</radio-button>
    <radio-button>Tea</radio-button>
    <radio-button>Cola</radio-button>
    <radio-button>Ginger Ale</radio-button>
  </radio-group>
</div>

<script src="https://cdn.rawgit.com/webcomponents/custom-elements/master/custom-elements.min.js"></script>

<script>
  class RadioButton extends HTMLElement {
    constructor() {
      super();
    }

    connectedCallback() {
      this.setAttribute('role', 'radio');
      this.setAttribute('tabindex', -1);
      this.setAttribute('aria-checked', false);
    }
  }

  window.customElements.define('radio-button', RadioButton);

  // Define values for keycodes
  const VK_LEFT       = 37;
  const VK_UP         = 38;
  const VK_RIGHT      = 39;
  const VK_DOWN       = 40;

  class RadioGroup extends HTMLElement {
    constructor() {
      super();
    }

    connectedCallback() {
      this.setAttribute('role', 'radiogroup');
      this.radios = Array.from(this.querySelectorAll('radio-button'));

      // Setup initial state
      if (this.hasAttribute('selected')) {
        let selected = this.getAttribute('selected');
        this._selected = selected;
        this.radios[selected].setAttribute('tabindex', 0);
        this.radios[selected].setAttribute('aria-checked', true);
      } else {
        this._selected = 0;
        this.radios[0].setAttribute('tabindex', 0);
      }

      this.addEventListener('keydown', this.handleKeyDown.bind(this));
      this.addEventListener('click', this.handleClick.bind(this));
    }

    handleKeyDown(e) {
      switch(e.keyCode) {

        case VK_UP:
        case VK_LEFT: {
          e.preventDefault();

          if (this.selected === 0) {
            this.selected = this.radios.length - 1;
          } else {
            this.selected--;
          }
          break;

        }

        case VK_DOWN:
        case VK_RIGHT: {
          e.preventDefault();

          if (this.selected === this.radios.length - 1) {
            this.selected = 0;
          } else {
            this.selected++;
          }
          break;
        }

      }
    }

    handleClick(e) {
      const idx = this.radios.indexOf(e.target);
      if (idx === -1) {
        return;
      }
      this.selected = idx;
    }

    set selected(idx) {
      if (isFinite(this.selected)) {
        // Set the old button to tabindex -1
        let previousSelected = this.radios[this.selected];
        previousSelected.tabIndex = -1;
        previousSelected.removeAttribute('aria-checked', false);
      }

      // Set the new button to tabindex 0 and focus it
      let newSelected = this.radios[idx];
      newSelected.tabIndex = 0;
      newSelected.focus();
      newSelected.setAttribute('aria-checked', true);

      this.setAttribute('selected', idx);
      this._selected = idx;
    }

    get selected() {
      return this._selected;
    }
  }

  window.customElements.define('radio-group', RadioGroup);
 </script>