Buddy Strings LeetCode solution in JavaScript

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var buddyStrings = function (A, B) {
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  if (A.length != B.length) return false;
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  const diff = [];
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  for (let i = 0; i < A.length; i++) {
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    if (A[i] != B[i]) diff.push(i);
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    if (diff.length > 2) return false;
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  }
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  if (!diff.length) return A.length != [...new Set(A)].length;
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  const [i, j] = diff;
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  return A[i] == B[j] && B[i] == A[j];
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};