这一点。$ 限制了jQuery的上下文视图的元素，所以操作速度更快。

this.$ limits jQuery's context to the view's element, so operations are quicker.

Additionaly，这一点。$（'。TODO项目'）将无法找到待办事项项的内容视图的元素之外类。

Additionaly, this.$('.todo-item') won't find your elements with todo-item class outside your view's element.