What's the best way to find the first common parent of two DOM nodes in javascript?

My question is exactly that but in context I want to examine the selection object, compare the anchorNode and focusNode and if they are different then find the first common parent element.

var selected = window.getSelection();
var anchor = selection.anchorNode;
var focus = selection.focusNode;

if ( anchor != focus ) {
   // find common parent...
}

I would try something like this, assuming no JS library:

function findFirstCommonAncestor(nodeA, nodeB, ancestorsB) {
    var ancestorsB = ancestorsB || getAncestors(nodeB);
    if(ancestorsB.length == 0) return null;
    else if(ancestorsB.indexOf(nodeA) > -1) return nodeA;
    else if(nodeA == document) return null;
    else return findFirstCommonAncestor(nodeA.parentNode, nodeB, ancestorsB);
}

using this utilities:

function getAncestors(node) {
    if(node != document) return [node].concat(getAncestors(node.parentNode));
    else return [node];
}

if(Array.prototype.indexOf === undefined) {
    Array.prototype.indexOf = function(element) {
        for(var i=0, l=this.length; i<l; i++) {
            if(this[i] == element) return i;
        }
        return -1;
    };
}

Then you can call findFirstCommonAncestor(myElementA, myElementB).