Distinct Subsequences LeetCode solution in JavaScript

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var numDistinct = function (s, t) {
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  const n = s.length;
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  const memo = {};
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  const dfs = (index = 0, subsequence = "") => {
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    if (subsequence === t) return 1;
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    if (n - index + 1 < t.length - subsequence.length) return 0;
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    if (index === n) return 0;
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    const key = `${index}-${subsequence}`;
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    if (key in memo) return memo[key];
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    memo[key] =
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      t[subsequence.length] !== s[index]
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        ? 0
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        : dfs(index + 1, subsequence + s[index]);
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    memo[key] += dfs(index + 1, subsequence);
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    return memo[key];
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  };
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  return dfs();
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};