老师看看我这个方法哪错了，是用来判断某条todo是否应该可见，代码如下：
isVisiable(i){
if (this.visibility === 'all') return true;
if (this.visibility === 'active') return !this.todos[i].done;
if (this.visibility === 'complete') return this.todos[i].done;
}
提示 TypeError: Cannot read property 'visibility' of undefined
