Domino and Tromino Tiling LeetCode solution in JavaScript

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var numTilings = function (n) {
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  let mod = 10 ** 9 + 7;
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  let len = 4;
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  let ways = new Array(len).fill(0);
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  // base cases
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  ways[0] = 1;
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  ways[1] = 1;
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  ways[2] = 2;
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  // already calculated above
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  if (n < len - 1) {
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    return ways[n];
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  }
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  // use % len to circulate values inside our array
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  for (var i = len - 1; i <= n; i++) {
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    ways[i % len] =
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      (ways[(len + i - 1) % len] * 2 + ways[(len + i - 3) % len]) % mod;
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  }
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  return ways[(i - 1) % len];
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};