How can I test which node comes first?

Question

I have two given nodes that are stored inside variables. Is there a simple, low resource usage, solution to find which node comes first in the document? Both nodes should be siblings but may be many nodes apart.

Answer 1

Try compareDocumentPosition:

function theFirst(node1, node2) {
  return node1.compareDocumentPosition(node2)
    & Node.DOCUMENT_POSITION_FOLLOWING ? node1 : node2;
}

Note that if the nodes are in different trees, the result may be random (but consistent). You can filter out that case with & Node.DOCUMENT_POSITION_DISCONNECTED and return e.g. undefined.