(function(l, l2, s) {

l2 = l2 || 6;

NodeList.prototype.forEach = [].forEach;
NodeList.prototype.slice = [].slice;

var toc = document.createElement('div');
var headerLevel = l || 2;
var tocStyle = s || 'ul';
var headers = document
  .querySelector('article')
  .querySelectorAll('h1 h2 h3 h4 h5 h6'.split(' ').slice(headerLevel - 1).join())

function buildList(header) {
  var li = document.createElement('li');
  var a = document.createElement('a');
  var a_ = header.querySelector('a');
  a.href = a_.href;
  a.innerHTML = header.innerText;
  li.appendChild(a);
  return li;
}

(function buildTocTree(tree, headers, level) {
  var i, m,
      indices = [],
      hx = 'H' + level,
      n = headers.length,
      ul = document.createElement(tocStyle);

  if(!n || level > l2) return;
  tree.appendChild(ul);

  function set(start, end) {
    var li = buildList(headers[start]);
    ul.appendChild(li);
    buildTocTree(li, headers.slice(start + 1, end), level + 1);
  }

  for(i = 0; i < n; ++i) if(headers[i].tagName === hx) indices.push(i);
  for(i = 0, m = indices.length - 1; i < m; ++i) set(indices[i], indices[i + 1]);
  if(indices.length) set(indices[m], n);
})(toc, headers, headerLevel);
console.log(toc.innerHTML);
})(2, 6, 'ul');