Find Longest Awesome Substring LeetCode solution in JavaScript

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var longestAwesome = function (s) {
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  // freq starts with 0:0 because 9 0s is also a state and if I come across a
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  // 0 down the road, that means that the whole array up to index i is of the required type
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  let firstIndex = { 0: 0 },
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    result = -1,
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    curr = 0;
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  for (let i = 0; i < s.length; i++) {
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    curr ^= 1 << s[i];
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    // Check if you have seen curr^0=curr before,
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    // because that would make the inbetween elements' xor = 000000000
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    if (firstIndex[curr] !== undefined)
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      result = Math.max(result, i - firstIndex[curr] + 1);
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    // Check all the other xors, because that would make
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    // the inbetween elements of the required type (100000000,010000000,......,000000001)
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    for (let j = 0; j < 10; j++) {
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      let ele = 1 << j;
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      if (firstIndex[curr ^ ele] !== undefined)
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        //i-firstIndex[curr^ele] because on freq I saved
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        //the smallest index where I last met curr^ele
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        result = Math.max(result, i - firstIndex[curr ^ ele] + 1);
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    }
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    if (firstIndex[curr] === undefined) firstIndex[curr] = i + 1; // +1 cos 0th place is for my 0 state
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  }
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  return result;
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};