Web作业：specific word count (index of )

统计文件中某一词语出现次数:

HTML:
    <p id="p1">start,stop,speed,start,speed ,velocicty,start</p>
    <button onclick="fun()">Try it</button>
    <p id="demo"></p>
    <script src="./index.js"></script>

JS:
function fun(){
    var str = document.getElementById("p1").innerHTML;
    var num=0;
    var a=str.indexOf("start");
    while(a!==-1){
        a=str.indexOf("start",a+1);
        num++;
    }
    document.getElementById("demo").innerHTML = num;
}