computed で classを動的に指定

html

index.html

<div id="app">
  <p :class="textStyle">test text</p>
</div>

css

style.css

.textColor{
  color: blue;
}

.textFont {
  font-size: 30px;
  background: red;
}

Vue.js

new Vue({
  el: "#app",
  data: function(){
      return {
          shouldApply: true
      }
  },
  computed: {
      textStyle: function() {
          return {
              textColor: this.shouldApply,
              textFont: this.shouldApply
          }
      }
  }
})

※computedはreturnとthisがいるよ