Maximum Length of a Concatenated String with Unique Characters LeetCode solution in JavaScript

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var maxLength = function (arr) {
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  const n = arr.length;
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  let maxLen = 0;
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  backtrack(0, "");
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  return maxLen;
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  function backtrack(start, prevStr) {
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    maxLen = Math.max(maxLen, prevStr.length);
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    if (start === n) return;
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    for (let i = start; i < n; i++) {
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      const word = arr[i];
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      if (isUnique(prevStr + word)) backtrack(i + 1, prevStr + word);
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    }
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  }
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  function isUnique(str) {
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    let bits = 0 | 0;
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    for (const char of str) {
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      const index = char.charCodeAt(0) - 97;
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      if (bits & (1 << index)) return false;
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      bits |= 1 << index;
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    }
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    return true;
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  }
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};