How do I make this loop all children recursively?

function allDescendants (node) {
    for (var i = 0; i < node.childNodes.length; i++) {
      var child = node.childNodes[i];
      allDescendants(child);
      doSomethingToNode(child);
    }
}

You loop over all the children, and for each element, you call the same function and have it loop over the children of that element.