详解vue中computed 和 watch的异同

<div id="myDiv">
  <input type="text" v-model="firstName">
  <input type="text" v-model="lastName">
  <input type="text" v-model="fullName">
</div>

 new Vue({
 el: '#myDiv',
 data: {
  firstName: 'Foo',
  lastName: 'Bar',
  fullName: 'Foo Bar'
 },
 watch: {
  firstName: function (val) {
   this.fullName = val + ' ' + this.lastName
  },
  lastName: function (val) {
   this.fullName = this.firstName + ' ' + val
  }
 }
})

 new Vue({
    el:"#myDiv",
      data:{
        firstName:"Den",
        lastName:"wang",

      },
      puted:{
        fullName:function(){
          return this.firstName + " " +this.lastName;
        }
      }
  })

puted:{
  fullName:{
  //这里用了es6书写方法
    set(){
       alert("set");
    },
    get(){
      alert("get");
      return this.firstName + " " +this.lastName;
    },

  }
 }