[Vue] mounted 에서 data undefined 발생할 경우

mounted : function() {
  let $vm = this
  this.$nextTick(function() {
    document.getElementById('btn1').addEventListener('click', function () {
      console.log(typeof($vm.items))
      console.log($vm.items)
      $vm.items.push('B')
    })
  });
},

아니면

arrow function 사용

출처 : https://stackoverflow.com/questions/51929737/vuejs-data-property-returns-undefined-in-mounted-function