var str = 'asssddnsbbs';
  let obj = {}, max = 0, maxValue = null;
  for (let i  of str){
      if(!obj[i]) obj[i] = 1;
      else obj[i]+=1;
  }
  for (let item in obj){
      if(obj[item]>max){
          max= obj[item]
      }
  }
  for (let item in obj){
      if(obj[item]===max){
          maxValue= item
      }
  }
  console.log(max, maxValue)