code: function TreeWalker(root, whatToShow, filter) {
    if (!root || !root.nodeType) {
      utils.NotSupportedError();
    }
  
    // Read-only properties
    this._root = root;
    this._whatToShow = Number(whatToShow) || 0;
    this._filter = filter || null;
    this._active = false;
    // Read-write property
    this._currentNode = root;
  }
docs: /* Public API 
  Latest version: https://dom.spec.whatwg.org/#treewalker
  
  @constructor
  @param {Node} root
  @param {number} whatToShow [optional]
  @param {Function|NodeFilter} filter [optional]
  @throws Error