Vowel count with if

vowelcount-withif.js

var withIf = function (s) {
  var vowels = 0;
  for (var i=0; i < s.length-1; i++){
    var letter = s[i].toLowerCase();
    if (letter === "a" || letter === "e" || letter === "i" || letter === "o" || letter === "u" ){
      vowels++;
    }
  }
  return vowels;
};