解决vue侦听器watch,调用this时出现undefined的问题

watch: {
 value: (newV, oldV) => {
 this.a = newV;
 }
}

watch: {
 value: function(newV, oldV) {
 this.a = newV;
 }
}

var app = new Vue({
   el: '#app',
   data: {
    a: 1
   },
 
   watch: {
    a:()=> {
     console.log(this)
    }
   },
 
   methods: {
    onClick() {
     this.a++;
    }
   } 
  })

"use strict"; 
var _this = void 0; 
var app = new Vue({
 el: '#app',
 data: {
 a: 1
 },
 watch: {
 a: function a() {
  console.log(_this);
 }
 },
 methods: {
 onClick: function onClick() {
  this.a++;
 }
 }
});

watch: { 
 s: function(newVal, old){ //非josn用法
   console.log(newVal)
  },
 'form.fdnDct': function(newVal, old){ //josn用法
   console.log(newVal)
  },
 }

 //这也是一种用法
 watch: {
  'browse_integral_info.buy':'RMB',     //购买积分转化成人民币
  'browse_integral_info.give':'GIVE',     //完成邀请浏览任务赠送
  'bargain_integral_info.give':'BARGAINGIVE',
  deep:true,
 },
 methods: {
  RMB: function(){ 
   this.RMBs = this.browse_integral_info.buy/100
  },
 },