JavaScript Date prototype 属性

<p id="demo">单击按钮来调用新的myMet()方法,并显示这个月的月份，,使用新的myProp方法</p>
<button onclick="myFunction()">点我</button>
<script>
  Date.prototype.myMet = function() {
    if (this.getMonth() == 0) {
      this.myProp = "一月"
    };
    if (this.getMonth() == 1) {
      this.myProp = "二月"
    };
    if (this.getMonth() == 2) {
      this.myProp = "三月"
    };
    if (this.getMonth() == 3) {
      this.myProp = "四月"
    };
    if (this.getMonth() == 4) {
      this.myProp = "五月"
    };
    if (this.getMonth() == 5) {
      this.myProp = "六月"
    };
    if (this.getMonth() == 6) {
      this.myProp = "七月"
    };
    if (this.getMonth() == 7) {
      this.myProp = "八月"
    };
    if (this.getMonth() == 8) {
      this.myProp = "九月"
    };
    if (this.getMonth() == 9) {
      this.myProp = "十月"
    };
    if (this.getMonth() == 10) {
      this.myProp = "十一月"
    };
    if (this.getMonth() == 11) {
      this.myProp = "十二月"
    };
  }

  function myFunction() {
    var d = new Date();
    d.myMet();
    var x = document.getElementById("demo");
    x.innerHTML = d.myProp;
  }
</script>