Edit in JSFiddle

var vm = new Vue({
  el: '#vue-instance',
  data: {
    x: 1
  },
  computed: {
    doubleX: function() {
      return this.x * 2;
    }
  }
});
<div id="vue-instance">
  <input type="number" v-model="x"> result: {{ doubleX }}
</div>

              
            
          
            
              

External resources loaded into this fiddle: