根据另一个文本框的值填充另一个文本框

<input type="text" id="input1" oninput="fillInput2()" />
<input type="text" id="input2" />

<script>
function fillInput2() {
  var input1Value = document.getElementById("input1").value;
  document.getElementById("input2").value = input1Value;
}
</script>

<div id="app">
  <input type="text" v-model="input1" />
  <input type="text" v-model="input2" />
</div>

<script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>
<script>
new Vue({
  el: '#app',
  data: {
    input1: '',
    input2: ''
  },
  watch: {
    input1: function(newValue) {
      this.input2 = newValue;
    }
  }
});
</script>