watcher in vuejs 2 code example

Example 1: vue watch

var vm = new Vue({
  el: '#demo',
  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
    }
  }
})

Example 2: vue change input value from console

let el = document.getElementById("id");
el.value = val;
el.dispatchEvent(new Event('input'));