问在keyDown事件上调整Vue.js文本区域的大小，如何获取样式属性？

EN

const style = window.getComputedStyle(this.$refs.textarea, null);
if (style.boxSizing === 'content-box') {
  heightOffset = -(parseFloat(style.paddingTop + parseFloat(style.paddingBottom));
} else {
  heightOffset = parseFloat(style.borderTopWidth) + parseFloat(style.borderBottomWidth);
}

<textarea 
  class="answer" 
  ref="textarea"
  v-bind:placeholder="placeholder" 
  v-on:keydown="setHeight"
  v-bind:style="{ height: areaHeight + 'px' }"
>
</textarea>

data: function () {
  return {
    placeholder: "Your answer",
      areaHeight: 24
  }
},
methods: {
  setHeight: function(e) {
    var ta = this.$refs.textarea;

    console.log("ta.paddingTop: " + ta.paddingTop); //undefined
    console.log("ta.style.paddingTop: " + ta.style.paddingTop); //""
    console.log("ta.paddingBottom: " + ta.paddingBottom); //undefined
    console.log("ta.style.paddingBottom: " + ta.style.paddingBottom); //""

    console.log("ta.borderTopWidth: " + ta.borderTopWidth); //undefined
    console.log("ta.style.borderTopWidth: " + ta.style.borderTopWidth); //""
    console.log("ta.borderBottomWidth: " + ta.borderBottomWidth); //undefined
    console.log("ta.style.borderBottomWidth: " + ta.style.borderBottomWidth); //""

    this.areaHeight = this.$refs.textarea.scrollHeight;

    if (e.keyCode === 13) {
      this.areaHeight += 24;
    }
    if (e.keyCode === 8) {
      this.areaHeight -= 24;
      if(this.areaHeight <= 0) this.areaHeight = 24;
    }
}