<p id="demo">单击按钮从位置20开始查找给定值最后出现的位置</p>

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

<script>

function myFunction(){

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

    var n=str.lastIndexOf("twle", 20);

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

}

</script>

​