Insert an HTML string before the start of the specified element

Use Element.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);

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

Source

https://www.30secondsofcode.org/js/s/insert-before