compareDocumentPosition.js

//Attach me to Node.prototype as well
var DOCUMENT_POSITION_DISCONNECTED = Node.prototype.DOCUMENT_POSITION_DISCONNECTED = 0x01
var DOCUMENT_POSITION_PRECEDING = Node.prototype.DOCUMENT_POSITION_PRECEDING = 0x02
var DOCUMENT_POSITION_FOLLOWING = Node.prototype.DOCUMENT_POSITION_FOLLOWING = 0x04
var DOCUMENT_POSITION_CONTAINS = Node.prototype.DOCUMENT_POSITION_CONTAINS = 0x08
var DOCUMENT_POSITION_CONTAINED_BY = Node.prototype.DOCUMENT_POSITION_CONTAINED_BY = 0x10

Node.prototype.compareDocumentPosition = function compareDocumentPosition( otherNode ) {
  if( !(otherNode instanceof Node) ) {
    throw Error("Comparing position against non-Node values is not allowed")
  }
  if( this === otherNode ) return 0
  if( this.ownerDocument !== otherNode.ownerDocument ) {
    return DOCUMENT_POSITION_DISCONNECTED
  }
  var point = this
  var parents = [ ]
  while( point ) {
    if( point == otherNode ) return DOCUMENT_POSITION_CONTAINS
    parents.push( point )
    point = point.parentNode
  }
  point = otherNode
  while( point ) {
    if( point == this ) return DOCUMENT_POSITION_CONTAINED_BY
    var location_index = parents.indexOf( point )
    if( location_index !== -1) {
		 var smallest_common_ancestor = parents[ location_index ]
		 var this_index = smallest_common_ancestor.childNodes.indexOf( this )
		 var other_index = smallest_common_ancestor.childNodes.indexOf( otherNode )
		 if( this_index > other_index ) {
           return DOCUMENT_POSITION_PRECEDING
		 }
		 else {
		   return DOCUMENT_POSITION_FOLLOWING
		 }
    }
    point = point.parentNode
  }
  return DOCUMENT_POSITION_DISCONNECTED
}