promise-dfs.js

function traverse(node) {
  return Promise.all(R.map(c => Promise.resolve(c), node.children))
  .then((nodes) => {
    return Promise.all(nodes.map(traverse))
    .then(R.reduce((prev, arr) => arr.concat(prev), []))
    .then(els => els.concat(nodes));
  });
}
traverse(document.body).then(R.map(e => console.log(e.tagName)));