reverse.js

//reverseVowels
//"whiteboard"
//"whatobeird"

const VOWELS = ['a','e','i','o','u'];

function isVowel(char) {
  return VOWELS.indexOf(char) >= 0;
}

function reverseVowels(string) {
    var half = Math.floor(string.length / 2);
    var i = 0, j = string.length - 1;

    while (i < half) {
      let a = string[i];
      let b = string[j];

      if (isVowel(a) && isVowel(b)) {
        string = string.substr(0, i) + b + string.substr(i + 1);
        string = string.substr(0, j) + a + string.substr(j + 1);
        ++i;
        --j;
      }

      if (!isVowel(a)) ++i;
      if (!isVowel(b)) --j;
    }

    return string;
}

console.log(reverseVowels('whiteboard'));