const arr = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30];


const binarySearch = (arr, x , start=0, end=arr.length) => {
  if(end < start) return -1;
  
  let mid = Math.floor((start + end) / 2);
  
  if(arr[mid] === x) return mid;

  if(arr[mid] < x) return binarySearch(arr, x, mid+1, end);
  
  else return binarySearch(arr, x , start, mid-1);
}

console.log(arr[binarySearch(arr,8)]);