Longest Substring Without Repeating Characters LeetCode solution in JavaScript

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var lengthOfLongestSubstring = function (s) {
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  // keeps track of the most recent index of each letter.
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  const seen = new Map();
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  // keeps track of the starting index of the current substring.
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  let start = 0;
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  // keeps track of the maximum substring length.
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  let maxLen = 0;
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  for (let i = 0; i < s.length; i++) {
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    // if the current char was seen, move the start to (1 + the last index of this char)
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    // max prevents moving backward, 'start' can only move forward
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    if (seen.has(s[i])) start = Math.max(seen.get(s[i]) + 1, start);
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    seen.set(s[i], i);
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    // maximum of the current substring length and maxLen
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    maxLen = Math.max(i - start + 1, maxLen);
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  }
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  return maxLen;
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};