检查属性是否有指定的

<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>