Count Unique Characters of All Substrings of a Given String LeetCode solution in JavaScript

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var uniqueLetterString = function (s) {
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  // Returns the number of unique characters inside the window [left, right]
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  const countUniqueCharacters = (left, right) => {
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    const counts = new Map();
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    for (let i = left; i < right; i += 1) {
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      counts.set(s[i], (counts.get(s[i]) || 0) + 1);
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    }
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    return Array.from(counts.values()).filter((x) => x === 1).length;
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  };
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  let sum = 0;
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  const seen = new Set();
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  const substring = (left, right) => {
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    if (seen.has(`${left}, ${right}`)) return;
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    if (right - left <= 0) return;
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    seen.add(`${left}, ${right}`);
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    sum += countUniqueCharacters(left, right);
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    for (let i = left; i < right; i += 1) {
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      substring(left + 1, right);
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      substring(left, right - 1);
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    }
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  };
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  substring(0, s.length);
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  return sum;
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};