elementContains

Returns true if the parent element contains the child element, false otherwise.

Check that parent is not the same element as child, use parent.contains(child) to check if the parent element contains the child element.

const elementContains = (parent, child) =>
  parent !== child && parent.contains(child);

elementContains(
  document.querySelector("head"),
  document.querySelector("title")
); // true
elementContains(document.querySelector("body"), document.querySelector("body")); // false