1. /**
    2.  * @param {string} s
    3.  * @param {number} k
    4.  * @return {number}
    5.  */
    6. var maxVowels = function(s, k) {
    7.   const target = new Set( ['a', 'e', 'i', 'o', 'u']),
    8.         length = s.length
    10.   let max = 0,
    11.       i = 0,
    12.       j = 0,
    13.       count = 0
    15.       while(j < k) {
    16.         if(target.has(s[j])) count++
    17.         j++
    18.       }
    20.       max = count
    22.       while(j <= length) {
    23.         if(target.has(s[j])) count++
    24.         if(target.has(s[i])) count--
    25.         j++
    26.         i++
    28.         max = Math.max(count, max)
    29.       }
    31.     return max
    32. };