SOURCE

var vm=new Vue({
  el:"#demo",
  data:{
    firstName:'Foo',
    lastName:'Bar',
  },
  computed：{
  fullName:function(){
  return this.firstName+' '+this.lastName
}
}
  // watch:{
  //   firstName:function(val){
  //     this.fullName=val+' '+this.lastName
  //   },
  //   lastName:function(val){
  //     this.fullName=this.firstName+' '+val
  //   }
  // }
})


var vm = new Vue({
  el: '#demo',
  data: {
    firstName: 'Foo',
    lastName: 'Bar'
  },
  computed: {
    fullName: function () {
      return this.firstName + ' ' + this.lastName
    }
  }
})

<div id='demo'>
  {{ fullName }}
</div>