Number of Good Ways to Split a String LeetCode solution in JavaScript

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var numSplits = function (s) {
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  let n = s.length;
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  let preFix = new Array(n),
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    suFix = new Array(n);
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  let preSet = new Set();
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  let suSet = new Set();
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  for (let i = 0; i < n; i++) {
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    preSet.add(s[i]);
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    suSet.add(s[n - 1 - i]);
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    preFix[i] = preSet.size;
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    suFix[n - 1 - i] = suSet.size;
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  }
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  let goodWays = 0;
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  for (let i = 0; i < n - 1; i++) {
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    if (preFix[i] === suFix[i + 1]) goodWays++;
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  }
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  return goodWays;
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};