var newNode = new Node(key); 运行时newNode = undifine;这是为什么？

function BinaryTree (){

var Node = function(key){

this.key   = key;

this.left  = null;

this.right = null;

};

var root = null;

var insertNode = function(node, newNode){

if (newNode.key < node.key) {

if (node.left === null) {

node.left = newNode;

}else {

insertNode(node.left, newNode);

}

} else {

if (node.right === null) {

node.right = newNode;

} else {

insertNode(node.right, newNode);

}

}

}

this.insert = function(key){

var newNode = new Node(key);

if (root === null) {

root = newNode;

} else {

insertNode(root, newNode);

}

}

}

var nodes = [8,3,56,1,9];

var binaryTree = new BinaryTree();

nodes.forEach(function(key){

binaryTree.insert(key);

});

var newNode = new Node(key);  运行时newNode = undifine;这是为什么？