Element: insertAdjacentElement() メソッド

insertAdjacentElement(position, element)

<!-- beforebegin -->
<p>
  <!-- afterbegin -->
  foo
  <!-- beforeend -->
</p>
<!-- afterend -->

beforeBtn.addEventListener("click", () => {
  const tempDiv = document.createElement("div");
  tempDiv.style.backgroundColor = randomColor();
  if (activeElem) {
    activeElem.insertAdjacentElement("beforebegin", tempDiv);
  }
  setListener(tempDiv);
});
afterBtn.addEventListener("click", () => {
  const tempDiv = document.createElement("div");
  tempDiv.style.backgroundColor = randomColor();
  if (activeElem) {
    activeElem.insertAdjacentElement("afterend", tempDiv);
  }
  setListener(tempDiv);
});