Longest Nice Substring LeetCode solution in JavaScript

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const swapCase = (str) =>
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  str
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    .split("")
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    .map((c) => (c === c.toUpperCase() ? c.toLowerCase() : c.toUpperCase()))
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    .join("");
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var longestNiceSubstring = function (s) {
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  let ans = "";
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  for (let i = 0; i < s.length; i++) {
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    for (let ii = i + 1; ii < s.length + 1; ii++) {
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      let substring = s.slice(i, ii); // we take a substring
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      let invertedCaseChars = [...substring].map(swapCase); // we create an array of chars from the substring and invert case of this chars
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      if (invertedCaseChars.every((char) => substring.includes(char))) {
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        // we check that substring includes every case inverted char (see the illustration above)
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        ans = substring.length > ans.length ? substring : ans; // we select the longest substring which satisfies our condition
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      }
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    }
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  }
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  return ans;
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};