promise-dfs.js

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

how did this magic happens?