class Node {
  constructor(val) {
    this._val = val;
    this._parent = null;
    this._children = [];
  }

  const isRoot = () => isValid(this._parent); // TODO: implement this
  const getchildren = () => this._children;
  const hasChildren = () => this._children.length > 0;
  const getvalue = () => this._val;
  const setvalue = (val) => this._val = val;
  const append = (child) => {
      child._parent = this;
      this._children.push(child);
      return this;
  }
  const toString = () => `Node (val: ${this._val}, children: ${this._children.length})`;
}

class Tree {
  constructor(root) {
    this._root = root;
  }


  static map(node, fn, tree = null) {
    node.value = fn(node.value);
    if(tree === null) {
      tree = new Tree(node);
    }

    if(node.hasChildren()) {
      _.map(node.children, function (child {
        Tree.map(child, fn, tree);
      }));
    }
    return tree;
  }

  const getroot = () => this._root;
}