Cannot read property of undefined error: Wrong class binding type for Vue bound object property

Question

I'm trying to bind a class based on an object property, but when I click on the list object to trigger the @click, the console tells me that the object property of isSelected cannot be read... I'm trying to do this on a dynamically rendered list item Use a function to receive the item's id and trigger a class to activate when this property is set to false or true

Hopefully someone can tell me what I'm missing in the code below, thanks :)

<ul class="daysList">
  <li v-for="day in January" v-bind:class="{addedToTrip: day.isSelected}" class="daynum" @click="toggleIsSelected(day)" :key= "day.id">
      {{day.val}}
  </li>
  </ul>

My vue data

data:function(){
    return{
      year: 2021,
      month: 1,
      day: 1,
      days:[18,19,20,21],
      January:[
          {name: 'January', isSelected: false, id: 1, val:1},
          {name: 'January', isSelected: false, id: 2, val:2},
          {name: 'January', isSelected: false, id: 3, val:3},
          {name: 'January', isSelected: false, id: 4, val:4},
          {name: 'January', isSelected: false, id: 5, val:5},
          {name: 'January', isSelected: false, id: 6, val:6},
          {name: 'January', isSelected: false, id: 7, val:7},
          {name: 'January', isSelected: false, id: 8, val:8},
          {name: 'January', isSelected: false, id: 9, val:9},
          {name: 'January', isSelected: false, id: 10, val:10},
          {name: 'January', isSelected: false, id: 11, val:11},
          {name: 'January', isSelected: false, id: 12, val:12},
          {name: 'January', isSelected: false, id: 13, val:13},
          {name: 'January', isSelected: false, id: 14, val:14},
          {name: 'January', isSelected: false, id: 15, val:15},
          {name: 'January', isSelected: false, id: 16, val:16},
          {name: 'January', isSelected: false, id: 17, val:17},
          {name: 'January', isSelected: false, id: 18, val:18},
          {name: 'January', isSelected: false, id: 19, val:19},
          {name: 'January', isSelected: false, id: 20, val:20},
          {name: 'January', isSelected: false, id: 21, val:21},
          {name: 'January', isSelected: false, id: 22, val:22},
          {name: 'January', isSelected: false, id: 23, val:23},
          {name: 'January', isSelected: false, id: 24, val:24},
          {name: 'January', isSelected: false, id: 25, val:25},
          {name: 'January', isSelected: false, id: 26, val:26},
          {name: 'January', isSelected: false, id: 27, val:27},
          {name: 'January', isSelected: false, id: 28, val:28},
          {name: 'January', isSelected: false, id: 29, val:29},
          {name: 'January', isSelected: false, id: 30, val:30},
          {name: 'January', isSelected: false, id: 31, val:31},

      ],
      February:[],
      March:[],
      April:[],
      May:[],
      June:[],
      July:[],
      August:[],
      September:[],
      October:[],
      November:[],
      December:[],
     

    }
  },

应用于 isSelected == true 时的 css 类

.addedToTrip{
    background-color: #2E9CFE;
    border-top: .5px solid black;
    border-bottom: .5px solid black;  
}

Answer 1

When specifying the class name, you need to use single quotes

ul class="daysList">
  <li v-for="day in January" v-bind:class="{'addedToTrip': day.isSelected}" class="daynum" @click="toggleIsSelected(day)" :key="day.id"> // 改变 added
      {{day.val}}
  </li>
  </ul>