Vowel doubler

vowelDoubler.js

/*
 * A function that takes an array of lowercase characters and returns
 * an array in which all of the vowels are repeated.
 * Time complexity: O(n) 
 * Space complexity: O(1)
 */
const vowelDoubler = function(arr) {
  let numVowels = 0;
  const vowels = new Set(['a', 'e', 'i', 'o', 'u']);
  for(let i = 0; i < arr.length; i++) {
    if(vowels.has(arr[i])) {
      numVowels += 1;
    }  
  } 
  
  let idx = arr.length - 1;
  arr = arr.concat(new Array(numVowels));
  
  while(idx >= 0) {
    arr[idx + numVowels] = arr[idx];
    if(vowels.has(arr[idx])) {
      numVowels -= 1;
      arr[idx + numVowels] = arr[idx];
    }
    idx -= 1;
  }
  return arr;
}

console.log(vowelDoubler(['a','d','e']))
console.log(vowelDoubler(['a']))
console.log(vowelDoubler(['d','y', 'r']))
console.log(vowelDoubler([]))