Given two DOM elements in the same document, find the closest common ancestor (which could be one of the elements).

findCommonParent.js

function findCommonParent(a, b) {
  var $a = $(a),
      $b = $(b),
      found;

  /*
   * A complicated one-liner here; with each element a & b,
   *   get the set of parents,
   *   and add to this the element
   *   .add() will sort the elements in document order though
   *     e.g. html, body, etc.
   *   which is the opposite of .parents()
   *   so use .get() to convert to an array,
   *   so we can .reverse()
   *   and wrap it in a $() to convert back to a jquery set
   *   to allow us to use .each()
   */
  $a = $($a.parents().add($a).get().reverse());
  $b = $($b.parents().add($b).get().reverse());
  $a.each(function(i1, p1) {
    $b.each(function(i2, p2) {
      if (p1 == p2) {
        found = p2;
        return false; // break $b.each()
      }
    });
    return ! found; // break $a.each() if found
  });

  return found;
}