Decode Ways LeetCode solution in JavaScript

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var numDecodings = function (s) {
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  let dp = Array(s.length).fill(0); // dp[i] means, the total ways of decode for substring up to i
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  dp[0] = s[0] !== "0" ? 1 : 0;
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  for (let i = 1; i < s.length; i++) {
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    //case1
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    if (s[i] !== "0") {
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      dp[i] = dp[i - 1];
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    }
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    //case2
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    if (s[i - 1] === "1" || (s[i - 1] === "2" && parseInt(s[i]) <= 6)) {
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      dp[i] += dp[i - 2] ?? 1;
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    }
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  }
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  return dp[s.length - 1];
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};