src/utils/iterate-focusable-elements.ts

/**
 * Options to the focusable elements iterator
 */
export interface IterateFocusableElements {
  /**
   * (Default: false) Iterate through focusable elements in reverse-order
   */
  reverse?: boolean

  /**
   * (Default: false) Perform additional checks to determine tabbability
   * which may adversely affect app performance.
   */
  strict?: boolean

  /**
   * (Default: false) Only iterate tabbable elements, which is the subset
   * of focusable elements that are part of the page's tab sequence.
   */
  onlyTabbable?: boolean
}

/**
 * Returns an iterator over all of the focusable elements within `container`.
 * Note: If `container` is itself focusable it will be included in the results.
 * @param container The container over which to find focusable elements.
 * @param reverse If true, iterate backwards through focusable elements.
 */
export function* iterateFocusableElements(
  container: HTMLElement,
  options: IterateFocusableElements = {}
): Generator<HTMLElement, undefined, undefined> {
  const strict = options.strict ?? false
  const acceptFn = options.onlyTabbable ?? false ? isTabbable : isFocusable
  const walker = document.createTreeWalker(container, NodeFilter.SHOW_ELEMENT, {
    acceptNode: node =>
      node instanceof HTMLElement && acceptFn(node, strict) ? NodeFilter.FILTER_ACCEPT : NodeFilter.FILTER_SKIP
  })
  let nextNode: Node | null = null

  // Allow the container to participate
  if (!options.reverse && acceptFn(container, strict)) {
    yield container
  }

  // If iterating in reverse, continue traversing down into the last child until we reach
  // a leaf DOM node
  if (options.reverse) {
    let lastChild = walker.lastChild()
    while (lastChild) {
      nextNode = lastChild
      lastChild = walker.lastChild()
    }
  } else {
    nextNode = walker.firstChild()
  }
  while (nextNode instanceof HTMLElement) {
    yield nextNode
    nextNode = options.reverse ? walker.previousNode() : walker.nextNode()
  }

  // Allow the container to participate (in reverse)
  if (options.reverse && acceptFn(container, strict)) {
    yield container
  }

  return undefined
}

/**
 * Determines whether the given element is focusable. If `strict` is true, we may
 * perform additional checks that require a reflow (less performant).
 * @param elem
 * @param strict
 */
export function isFocusable(elem: HTMLElement, strict = false): boolean {
  // Certain conditions cause an element to never be focusable, even if they have tabindex="0"
  const disabledAttrInert =
    ['BUTTON', 'INPUT', 'SELECT', 'TEXTAREA', 'OPTGROUP', 'OPTION', 'FIELDSET'].includes(elem.tagName) &&
    (elem as HTMLElement & {disabled: boolean}).disabled
  const hiddenInert = elem.hidden
  const hiddenInputInert = elem instanceof HTMLInputElement && elem.type === 'hidden'
  if (disabledAttrInert || hiddenInert || hiddenInputInert) {
    return false
  }

  // Each of the conditions checked below require a reflow, thus are gated by the `strict`
  // argument. If any are true, the element is not focusable, even if tabindex is set.
  if (strict) {
    const sizeInert = elem.offsetWidth === 0 || elem.offsetHeight === 0
    const visibilityInert = ['hidden', 'collapse'].includes(getComputedStyle(elem).visibility)
    const clientRectsInert = elem.getClientRects().length === 0
    if (sizeInert || visibilityInert || clientRectsInert) {
      return false
    }
  }

  // Any element with `tabindex` explicitly set can be focusable, even if it's set to "-1"
  if (elem.getAttribute('tabindex') != null) {
    return true
  }

  // One last way `elem.tabIndex` can be wrong.
  if (elem instanceof HTMLAnchorElement && elem.getAttribute('href') == null) {
    return false
  }

  return elem.tabIndex !== -1
}

/**
 * Determines whether the given element is tabbable. If `strict` is true, we may
 * perform additional checks that require a reflow (less performant). This check
 * ensures that the element is focusable and that its tabindex is not explicitly
 * set to "-1" (which makes it focusable, but removes it from the tab order).
 * @param elem
 * @param strict
 */
export function isTabbable(elem: HTMLElement, strict = false): boolean {
  return isFocusable(elem, strict) && elem.getAttribute('tabindex') !== '-1'
}