code: function (key, value) {
    this.left = null;
    this.right = null;
    this.height = null;
    this.key = key;
    this.value = value;
  }
docs: Creates a new AVL Tree node.
  
  @private
  @param {Object} key The key of the new node.
  @param {Object} value The value of the new node.