Find Missing Observations LeetCode solution in JavaScript

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var missingRolls = function (rolls, mean, n) {
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  const diff = mean * (rolls.length + n) - rolls.reduce((a, b) => a + b);
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  if (diff < n) return []; // Cannot be covered even by case 1 1 1 ...
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  if (diff > 6 * n) return []; // Cannot be covered even by case 6 6 6 ...
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  return Array(n)
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    .fill(Math.ceil(diff / n), 0, diff % n)
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    .fill(Math.floor(diff / n), diff % n);
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};