Inserts an HTML string before the start of the specified element.
- Use 
Element.insertAdjacentHTML()with a position of'beforebegin'to parsehtmlStringand insert it before the start ofel. 
代码实现
const insertBefore = (el, htmlString) =>
  el.insertAdjacentHTML('beforebegin', htmlString);
insertBefore(document.getElementById('myId'), '<p>before</p>');
// <p>before</p> <div id="myId">...</div>
翻译自:https://www.30secondsofcode.org/js/s/insert-html-string-before-element