insertBefore

Inserts an HTML string before the start of the specified element.

Use el.insertAdjacentHTML() with a position of 'beforebegin' to parse htmlString and insert it before the start of el.

const insertBefore = (el, htmlString) =>
  el.insertAdjacentHTML("beforebegin", htmlString);

insertBefore(document.getElementById("myId"), "<p>before</p>");
// <p>before</p> <div id="myId">...</div>