计算属性

 <div id="app">
        {{total}}
   </div>
   <script>
       var vm = new Vue({
           el:"#app",
           data(){
               return {
                   firstName:"zhang",
                   lastName:"san"
               }
           },
           /*当参与计算的值发生改变时,总值会改变 */
           computed:{
               total(){
                   return this.firstName + this.lastName
               }
           }
       })
   </script>