Longest Arithmetic Subsequence of Given Difference LeetCode solution in JavaScript

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var longestSubsequence = function (arr, difference) {
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  const map = new Map();
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  let max = 0;
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  for (let num of arr) {
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    const prev = map.has(num - difference) ? map.get(num - difference) : 0;
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    const val = prev + 1;
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    map.set(num, val);
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    max = Math.max(max, val);
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  }
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  return max;
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};