SOURCE

// [2,1,6,3,3,4]=>[1,3,4,6,3,2]
let string="helloworldddddddddd"
function demo(string){
 let arr=string.split("");
  let newArr=arr.sort()
  let temp=0;
  let who;
  let index=0;
  for(let i=0;i<newArr.length;i++){
    if(arr[i]==arr[i+1]){
      index++;
    }
    if(arr[i]!==arr[i+1]){
      if(temp<index){
        temp=index+1;
        who=arr[i];
      }
      else{
        temp=temp;
      }
      index=0;
    }
  }
  console.log(who,temp)
}
console.log(demo(string))