The contains() method of the Node interface returns a boolean value indicating whether a node is a descendant of a given node, that is the node itself, one of its direct children (childNodes), one of the children's direct children, and so on.

otherNode

The Node to test with.

Note: otherNode is not optional, but can be set to null.

A boolean value that is true if otherNode is contained in the node, false if not.

If the otherNode parameter is null, contains() always returns false.

This function checks to see if an element is in the page's body. As contains is inclusive and determining if the body contains itself isn't the intention of isInPage this case explicitly returns false.

function isInPage(node) {
  return node === document.body ? false : document.body.contains(node);
}

• Node.compareDocumentPosition
• Node.hasChildNodes