Vue3: Understanding ref vs. reactive

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import { customRef } from "vue"const focusableElementsSelector =  'button, [href], input, select, textarea, [tabindex]:not([tabindex="-1"])'const useFocusTrap = () => {  let focusableElements = []  let $firstFocusable  let $lastFocusable  const trapRef = customRef((track, trigger) => {    let $trapEl = null    return {      get() {        track()        return $trapEl      },      set(value) {        $trapEl = value        value ? initFocusTrap() : clearFocusTrap()        trigger()      }    }  })  function keyHandler(e) {    const isTabPressed = e.key === "Tab"    if (!isTabPressed) return    if (e.shiftKey) {      if (document.activeElement === $firstFocusable) {        $lastFocusable.focus()        e.preventDefault()      }    } else {      if (document.activeElement === $lastFocusable) {        $firstFocusable.focus()        e.preventDefault()      }    }  }  function initFocusTrap() {    // Bail out if there is no value    if (!trapRef.value) return    focusableElements = trapRef.value.querySelectorAll(      focusableElementsSelector    )    $firstFocusable = focusableElements[0]    $lastFocusable = focusableElements[focusableElements.length - 1]    document.addEventListener("keydown", keyHandler)    $firstFocusable.focus()  }  function clearFocusTrap() {    document.removeEventListener("keydown", keyHandler)  }  return {    trapRef,    initFocusTrap,    clearFocusTrap  }}export default useFocusTrap