Simple Binary Search Tree.js

function Node(key) {
  this.key = key;
  this.left = null;
  this.right = null;
  this.parent = null;
}

function BSTree() {
  this.root = null;
}

BSTree.prototype.insert = function (key) {
  if (this.root === null) {
    this.root = new Node(key);
    return;
  }
  this.insertChild(key, this.root);
};

BSTree.prototype.insertChild = function (key, node) {
  // find the spot to insert
  if(node.key > key) {
    if(node.left === null) {
      node.left = new Node(key);
      node.left.parent = node;
      return;
    } else {
      this.insertChild(key, node.left);
    }
  } else {
    if(node.right === null) {
      node.right = new Node(key);
      node.right.parent = node;
      return;
    } else {
      this.insertChild(key, node.right);
    }
  }
}

BSTree.prototype.build = function (keys, node) {
  for (var i = 0; i < keys.length; i++) {
    this.insert(keys[i], this.root);
  }
};

BSTree.prototype.contains = function (key, anode, path) {
  var node = anode ? anode : this.root;
  if (!node) {
    return false;
  }

  if (node.key === key) {
    path.push(node.key); // found
    return true;
  }
  
  if (node.left === null && node.right === null) {
    path.push('not found');
    return false;
  }
  
  path.push(node.key);
  
  if (node.key > key) {
    return this.contains(key, node.left, path);
  } else {
    return this.contains(key, node.right, path);
  }
};

let aTree = new BSTree();
aTree.build([5, 6, 8, 3, 7, 9, 10]);

var path = [];
// aTree.contains(11, null, path); // 5,6,8,9,not found

aTree.contains(8, null, path); // 5,6,8