Vowel Count With Binary Search

vowelcount-withbinary.js

var withBinarySearch = function (s) {
  var vowels = 0;
  var vowelArray = ["a","e","i","o","u"]; //already sorted
  for (var i = 0; i < s.length-1; i++){
    var search = binarySearch(vowelArray,s[i].toLowerCase(),0,4);
    if ( search !== -1 ){
      vowels++;
    }
  }
  return vowels;
};

/*
* Utility function
*/

function binarySearch (a,value,lo,hi) {
  if (lo>hi) {
    return -1;
  }
  var mid = Math.round((lo+hi)/2);
  if (value < a[mid]) {
    return binarySearch(a,value,lo,mid-1);
  }
  else if (value > a[mid]) {
    return binarySearch(a,value,mid+1,hi);
  }
  else if (value === a[mid]) {
    return mid;
  }
}