查找字符串 "mifengjc" 最后出现的位置

<p id="demo">单击按钮显示指定值最后一次出现的位置。</p>

<button onclick="myFunction()">点我</button>

<script>

  function myFunction() {

    var str = "I am from mifengjc，welcome to mifengjc site.";

    var n = str.lastIndexOf("mifengjc");

    document.getElementById("demo").innerHTML = n;

  }

</script>