this(wrong).js

TodoListManager.prototype.onItemChecked = function(event) {
  // Do something when the Todo item is clicked.
 var checked = event.target.classList.contains('checked');
 var todoId = Number.parseInt(event.target.dataset.todoId, 10);
 this._listTodoItem[todoId].checked = checked;
};

TodoListManager.prototype.start = function() {
  window.addEventListener('click', this.onItemChecked)
};

Put why the 9th line is wrong in comments.

是因為this嗎??

這個this是指Window，但是此處this應該是要指TodoListManager.