focus-visible.js

/**
 * Applies the :focus-visible polyfill at the given scope.
 * A scope in this case is either the top-level Document or a Shadow Root.
 *
 * @param {(Document|ShadowRoot)} scope
 * @see https://github.com/WICG/focus-visible
 */
function applyFocusVisiblePolyfill(scope) {
  var hadKeyboardEvent = true;
  var hadFocusVisibleRecently = false;
  var hadFocusVisibleRecentlyTimeout = null;

  var inputTypesAllowlist = {
    text: true,
    search: true,
    url: true,
    tel: true,
    email: true,
    password: true,
    number: true,
    date: true,
    month: true,
    week: true,
    time: true,
    datetime: true,
    'datetime-local': true
  };

  /**
   * Helper function for legacy browsers and iframes which sometimes focus
   * elements like document, body, and non-interactive SVG.
   * @param {Element} el
   */
  function isValidFocusTarget(el) {
    if (
      el &&
      el !== document &&
      el.nodeName !== 'HTML' &&
      el.nodeName !== 'BODY' &&
      'classList' in el &&
      'contains' in el.classList
    ) {
      return true;
    }
    return false;
  }

  /**
   * Computes whether the given element should automatically trigger the
   * `focus-visible` class being added, i.e. whether it should always match
   * `:focus-visible` when focused.
   * @param {Element} el
   * @return {boolean}
   */
  function focusTriggersKeyboardModality(el) {
    var type = el.type;
    var tagName = el.tagName;

    if (tagName === 'INPUT' && inputTypesAllowlist[type] && !el.readOnly) {
      return true;
    }

    if (tagName === 'TEXTAREA' && !el.readOnly) {
      return true;
    }

    if (el.isContentEditable) {
      return true;
    }

    return false;
  }

  /**
   * Add the `focus-visible` class to the given element if it was not added by
   * the author.
   * @param {Element} el
   */
  function addFocusVisibleClass(el) {
    if (el.classList.contains('focus-visible')) {
      return;
    }
    el.classList.add('focus-visible');
    el.setAttribute('data-focus-visible-added', '');
  }

  /**
   * Remove the `focus-visible` class from the given element if it was not
   * originally added by the author.
   * @param {Element} el
   */
  function removeFocusVisibleClass(el) {
    if (!el.hasAttribute('data-focus-visible-added')) {
      return;
    }
    el.classList.remove('focus-visible');
    el.removeAttribute('data-focus-visible-added');
  }

  /**
   * If the most recent user interaction was via the keyboard;
   * and the key press did not include a meta, alt/option, or control key;
   * then the modality is keyboard. Otherwise, the modality is not keyboard.
   * Apply `focus-visible` to any current active element and keep track
   * of our keyboard modality state with `hadKeyboardEvent`.
   * @param {KeyboardEvent} e
   */
  function onKeyDown(e) {
    if (e.metaKey || e.altKey || e.ctrlKey) {
      return;
    }

    if (isValidFocusTarget(scope.activeElement)) {
      addFocusVisibleClass(scope.activeElement);
    }

    hadKeyboardEvent = true;
  }

  /**
   * If at any point a user clicks with a pointing device, ensure that we change
   * the modality away from keyboard.
   * This avoids the situation where a user presses a key on an already focused
   * element, and then clicks on a different element, focusing it with a
   * pointing device, while we still think we're in keyboard modality.
   * @param {Event} e
   */
  function onPointerDown(e) {
    hadKeyboardEvent = false;
  }

  /**
   * On `focus`, add the `focus-visible` class to the target if:
   * - the target received focus as a result of keyboard navigation, or
   * - the event target is an element that will likely require interaction
   *   via the keyboard (e.g. a text box)
   * @param {Event} e
   */
  function onFocus(e) {
    // Prevent IE from focusing the document or HTML element.
    if (!isValidFocusTarget(e.target)) {
      return;
    }

    if (hadKeyboardEvent || focusTriggersKeyboardModality(e.target)) {
      addFocusVisibleClass(e.target);
    }
  }

  /**
   * On `blur`, remove the `focus-visible` class from the target.
   * @param {Event} e
   */
  function onBlur(e) {
    if (!isValidFocusTarget(e.target)) {
      return;
    }

    if (
      e.target.classList.contains('focus-visible') ||
      e.target.hasAttribute('data-focus-visible-added')
    ) {
      // To detect a tab/window switch, we look for a blur event followed
      // rapidly by a visibility change.
      // If we don't see a visibility change within 100ms, it's probably a
      // regular focus change.
      hadFocusVisibleRecently = true;
      window.clearTimeout(hadFocusVisibleRecentlyTimeout);
      hadFocusVisibleRecentlyTimeout = window.setTimeout(function() {
        hadFocusVisibleRecently = false;
      }, 100);
      removeFocusVisibleClass(e.target);
    }
  }

  /**
   * If the user changes tabs, keep track of whether or not the previously
   * focused element had .focus-visible.
   * @param {Event} e
   */
  function onVisibilityChange(e) {
    if (document.visibilityState === 'hidden') {
      // If the tab becomes active again, the browser will handle calling focus
      // on the element (Safari actually calls it twice).
      // If this tab change caused a blur on an element with focus-visible,
      // re-apply the class when the user switches back to the tab.
      if (hadFocusVisibleRecently) {
        hadKeyboardEvent = true;
      }
      addInitialPointerMoveListeners();
    }
  }

  /**
   * Add a group of listeners to detect usage of any pointing devices.
   * These listeners will be added when the polyfill first loads, and anytime
   * the window is blurred, so that they are active when the window regains
   * focus.
   */
  function addInitialPointerMoveListeners() {
    document.addEventListener('mousemove', onInitialPointerMove);
    document.addEventListener('mousedown', onInitialPointerMove);
    document.addEventListener('mouseup', onInitialPointerMove);
    document.addEventListener('pointermove', onInitialPointerMove);
    document.addEventListener('pointerdown', onInitialPointerMove);
    document.addEventListener('pointerup', onInitialPointerMove);
    document.addEventListener('touchmove', onInitialPointerMove);
    document.addEventListener('touchstart', onInitialPointerMove);
    document.addEventListener('touchend', onInitialPointerMove);
  }

  function removeInitialPointerMoveListeners() {
    document.removeEventListener('mousemove', onInitialPointerMove);
    document.removeEventListener('mousedown', onInitialPointerMove);
    document.removeEventListener('mouseup', onInitialPointerMove);
    document.removeEventListener('pointermove', onInitialPointerMove);
    document.removeEventListener('pointerdown', onInitialPointerMove);
    document.removeEventListener('pointerup', onInitialPointerMove);
    document.removeEventListener('touchmove', onInitialPointerMove);
    document.removeEventListener('touchstart', onInitialPointerMove);
    document.removeEventListener('touchend', onInitialPointerMove);
  }

  /**
   * When the polfyill first loads, assume the user is in keyboard modality.
   * If any event is received from a pointing device (e.g. mouse, pointer,
   * touch), turn off keyboard modality.
   * This accounts for situations where focus enters the page from the URL bar.
   * @param {Event} e
   */
  function onInitialPointerMove(e) {
    // Work around a Safari quirk that fires a mousemove on <html> whenever the
    // window blurs, even if you're tabbing out of the page. ¯\_(ツ)_/¯
    if (e.target.nodeName && e.target.nodeName.toLowerCase() === 'html') {
      return;
    }

    hadKeyboardEvent = false;
    removeInitialPointerMoveListeners();
  }

  // For some kinds of state, we are interested in changes at the global scope
  // only. For example, global pointer input, global key presses and global
  // visibility change should affect the state at every scope:
  document.addEventListener('keydown', onKeyDown, true);
  document.addEventListener('mousedown', onPointerDown, true);
  document.addEventListener('pointerdown', onPointerDown, true);
  document.addEventListener('touchstart', onPointerDown, true);
  document.addEventListener('visibilitychange', onVisibilityChange, true);

  addInitialPointerMoveListeners();

  // For focus and blur, we specifically care about state changes in the local
  // scope. This is because focus / blur events that originate from within a
  // shadow root are not re-dispatched from the host element if it was already
  // the active element in its own scope:
  scope.addEventListener('focus', onFocus, true);
  scope.addEventListener('blur', onBlur, true);

  // We detect that a node is a ShadowRoot by ensuring that it is a
  // DocumentFragment and also has a host property. This check covers native
  // implementation and polyfill implementation transparently. If we only cared
  // about the native implementation, we could just check if the scope was
  // an instance of a ShadowRoot.
  if (scope.nodeType === Node.DOCUMENT_FRAGMENT_NODE && scope.host) {
    // Since a ShadowRoot is a special kind of DocumentFragment, it does not
    // have a root element to add a class to. So, we add this attribute to the
    // host element instead:
    scope.host.setAttribute('data-js-focus-visible', '');
  } else if (scope.nodeType === Node.DOCUMENT_NODE) {
    document.documentElement.classList.add('js-focus-visible');
    document.documentElement.setAttribute('data-js-focus-visible', '');
  }
}

// It is important to wrap all references to global window and document in
// these checks to support server-side rendering use cases
// @see https://github.com/WICG/focus-visible/issues/199
if (typeof window !== 'undefined' && typeof document !== 'undefined') {
  // Make the polyfill helper globally available. This can be used as a signal
  // to interested libraries that wish to coordinate with the polyfill for e.g.,
  // applying the polyfill to a shadow root:
  window.applyFocusVisiblePolyfill = applyFocusVisiblePolyfill;

  // Notify interested libraries of the polyfill's presence, in case the
  // polyfill was loaded lazily:
  var event;

  try {
    event = new CustomEvent('focus-visible-polyfill-ready');
  } catch (error) {
    // IE11 does not support using CustomEvent as a constructor directly:
    event = document.createEvent('CustomEvent');
    event.initCustomEvent('focus-visible-polyfill-ready', false, false, {});
  }

  window.dispatchEvent(event);
}

if (typeof document !== 'undefined') {
  // Apply the polyfill to the global document, so that no JavaScript
  // coordination is required to use the polyfill in the top-level document:
  applyFocusVisiblePolyfill(document);
}