HTML DOM specified 属性

<p id="demo">单击按钮查找按钮是否有onclick属性</p>
<button onclick="myFunction()">点我</button>
<script>
  function myFunction() {
    var btn = document.getElementsByTagName("BUTTON")[0];
    var x = document.getElementById("demo");
    x.innerHTML = btn.getAttributeNode("onclick").specified;
  }
</script>