Detect identical nodes using the node's position

tree-walker.js

// Ignore whitespace && body nodes
function shouldIgnoreNode(node) {
    return (node.nodeName === 'BODY' || node.nodeType === 8 || node.nodeType === 3);
}

// Safely walk up previousSibling nodes by ignoring whitespace nodes
function getPrevNode(node) {
    var sibling = node.previousSibling;

    if (!sibling) {
        return null;
    }

    if (shouldIgnoreNode(sibling)) {
        return getPrevNode(sibling);
    }

    return sibling;
}

// Given a node, return the position as an array where 
// the keys equal the nodes position in the tree relative to siblings/parents
function parse(node) {
    var rtn = [];
    var count = 0;

    if (shouldIgnoreNode(node)) {
        return rtn;
    }

    var prevSibling = node.previousSibling;
    var parentNode = node.parentNode;

    if (prevSibling !== null) {
        while(prevSibling = getPrevNode(prevSibling)) {
            count++;
        }
        rtn.push(count);
    }

    if (!prevSibling && parentNode !== null) {
        rtn = rtn.concat(parse(parentNode));
    }

    return rtn;
}

// Given a target node, generate an array indicating its position relative
// to the root node. Reverse the output so we can 'climb' back down the tree
function buildTreeMap(element) {
    var treeMap = parse(element);
    return treeMap.reverse();
}

var element = document.getElementById('me');
var map = buildTreeMap(element);

console.log(map);