javascript binary search

const arr = [1, 3, 5, 7, 8, 9];
const binarySearch = (arr, x , start=0, end=arr.length) => {
  // If the item does not exist, return -1
  if(end < start) return -1;
  
  // Calculate middle index of the array
  let mid = Math.floor((start + end) / 2);
  
  // Is the middle a match?
  if(arr[mid] === x) return mid;
  // Is the middle less than x
  if(arr[mid] < x) return binarySearch(arr, x, mid+1, end);
  // Else the middle is more than x
  else return binarySearch(arr, x , start, mid-1);
}

binarySearch(arr, 7); // Returns 3
/* O(n) of binarySearch is log(n) */