roll-focus.js

function rollFocus(backFlag) {
  let focusIndex
  const currentIndex = this.tabbableElements.indexOf(document.activeElement)
  if (currentIndex < 0) {
    focusIndex = 0
  } else {
    const tabbableLength = this.tabbableElements.length
    const direction = backFlag ? -1 : 1
    focusIndex = (currentIndex + direction + tabbableLength) % tabbableLength
  }
  this.focus(this.tabbableElements[focusIndex])
}

function focus(element) {
  if (!element) {
    return
  }
  element.focus()
}