Find the Longest Substring Containing Vowels in Even Counts LeetCode solution in JavaScript

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var findTheLongestSubstring = function (s) {
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  // u o i e a
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  // 0 0 0 0 0 => initial state, all are even letters
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  // s = "abcab"
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  // 0 0 0 0 1 => at index 0, only a is odd count
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  // 0 0 0 0 1 => at index 1, max = 1
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  // 0 0 0 0 1 => at index 2, max = 2
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  // 0 0 0 0 0 => at index 3, max = 4
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  // 0 0 0 0 0 => at index 4, max = 5
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  // valid condition: same state in previous index, then it means we have a even count for all letters within the middle substring.
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  var mask = 0;
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  var t = "aeiou";
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  var count = new Map(); // <mask, first_idx>
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  count.set(0, -1);
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  var res = 0;
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  for (var i = 0; i < s.length; i++) {
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    if (t.indexOf(s[i]) >= 0) {
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      var j = t.indexOf(s[i]);
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      mask = mask ^ (1 << j);
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    }
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    if (!count.has(mask)) {
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      count.set(mask, i);
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    } else {
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      // substring is from [prevIdx+1, i];
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      res = Math.max(res, i - count.get(mask));
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    }
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  }
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  return res;
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};