How do you toggle a class in Vue.js?

For Vue.js 2 you could have the active class be dependent upon a boolean data value:

<th 
  class="initial" 
  v-on:click="myFilter"
  v-bind:class="{ active: isActive }"
>
  <span class="wkday">M</span>
</th>

methods: {
     myFilter: function(){          
        this.isActive = !this.isActive;
        // some code to filter users
    }
}