simple binary search example ...

bsearch.js

const bsearch = (arr , start , end , el) => {
  if(start > end) return -1 
  let mid = Math.round((start + end) / 2) 
  if(mid == el) return mid 
  else if ( mid < el ) return bsearch(arr , mid + 1 , end , el)
  else if ( mid > el ) return bsearch(arr , start , mid - 1 , el)
}