JS實現如何的統計一個字符串中相同的字符個數

         var string="start,stop,speed,start,speed,start,relocicty,start,start,start,start";
         var count=1;
         for (let index = 0; index < string.length; index++) {
            var a=string.indexOf("start",index);
             // console.log(a);
            if(a!=-1&&string.indexOf("start",index)!=string.indexOf("start",index-1)){
                 count++;
            }
         }
         console.log(count);