Dice Roll Simulation LeetCode solution in JavaScript

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/**
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 * @param {number} n
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 * @param {number[]} rollMax
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 * @return {number}
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 */
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var dieSimulator = function (n, rollMax) {
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  let dp = {};
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  let help = (prev, rolls, rollsLeft) => {
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    if (rolls == n) return 1;
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    if (dp[rolls] == undefined) dp[rolls] = {};
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    if (dp[rolls][rollsLeft] == undefined) dp[rolls][rollsLeft] = {};
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    if (dp[rolls][rollsLeft][prev] != undefined)
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      return dp[rolls][rollsLeft][prev];
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    let sum = 0;
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    for (let i = 0; i < rollMax.length; i++) {
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      if (rollsLeft === 0 && prev === i) continue;
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      if (prev === i) sum += help(i, rolls + 1, rollsLeft - 1);
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      else sum += help(i, rolls + 1, rollMax[i] - 1);
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    }
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    dp[rolls][rollsLeft][prev] = sum % (10 ** 9 + 7);
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    return dp[rolls][rollsLeft][prev];
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  };
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  return help(null, 0, Infinity);
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};