getSiblings

Returns an array containing all the siblings of the given element.

• Use Node.parentNode and Node.childNodes to get a NodeList of all the elements contained in the element's parent.
• Use the spread operator (...) and Array.prototype.filter() to convert to an array and remove the given element from it.

const getSiblings = el =>
  [...el.parentNode.childNodes].filter(node => node !== el);

getSiblings(document.querySelector('head')); // ['body']