Longest Arithmetic Subsequence LeetCode solution in JavaScript

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var longestArithSeqLength = function (nums) {
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  if (nums === null || nums.length === 0) {
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    return 0;
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  }
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  let diffMap = new Array(nums.length).fill(0).map(() => new Map());
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  let maxLen = 1;
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  for (let i = 0; i < nums.length; i++) {
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    for (let j = 0; j < i; j++) {
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      let diff = nums[i] - nums[j];
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      // if prev element has an ongoing arithmetic sequence with the same common difference
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      // we simply add 1 to the length of that ongoing sequence, hence diffMap[j].get(diff) + 1
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      // else, we start a new arithmetic sequence, initial length is 2
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      diffMap[i].set(diff, diffMap[j].get(diff) + 1 || 2);
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      maxLen = Math.max(maxLen, diffMap[i].get(diff));
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    }
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  }
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  return maxLen;
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  // T.C: O(N^2)
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  // S.C: O(N^2)
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};