JSFiddle - React, Tailwind, and code Playground

HTML

<div id="mainapp">
      <span class="star" v-bind:class="{gold:obj.selected}" v-on:click="clickStar()">star</span>
    </div>

CSS

.star{
  color:#888;
}
.star.gold{
  color:#FFE957;
}

JavaScript

var app = new Vue({
      el:"#mainapp",
      data:{
        obj:{}  
      },
      methods:{
        clickStar:function(){
          if(this.obj.selected == undefined) this.obj.selected =false;
          //this.obj.selected=!this.obj.selected;
          this.$set(this.obj, 'selected', !this.obj.selected);
          console.log(this.obj);
        }
      }
    })