Longest Repeating Character Replacement LeetCode solution in JavaScript

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// O(n) time | O(26) -> O(1) space - only uppercase English letters
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var characterReplacement = function (s, k) {
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  const sLen = s.length,
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    charCount = {};
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  if (k >= sLen) return sLen;
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  let maxLen = 0,
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    windowStart = 0,
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    maxRepeatChar = 0;
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  for (let windowEnd = 0; windowEnd < sLen; windowEnd++) {
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    // increment charCount
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    charCount[s[windowEnd]]
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      ? charCount[s[windowEnd]]++
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      : (charCount[s[windowEnd]] = 1);
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    // calc max repeating char
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    maxRepeatChar = Math.max(maxRepeatChar, charCount[s[windowEnd]]);
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    // calc number of char that is not (or has fewer chars) repeating in window
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    const remainingChar = windowEnd - windowStart + 1 - maxRepeatChar;
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    // slide window by incrementing start of window
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    if (remainingChar > k) {
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      // decrement charCount
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      charCount[s[windowStart]]--;
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      windowStart++;
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    }
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    // calc maxLen
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    maxLen = Math.max(maxLen, windowEnd - windowStart + 1);
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  }
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  return maxLen;
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};