Number of Ways to Select Buildings LeetCode solution in JavaScript

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var numberOfWays = function (s) {
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  const len = s.length;
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  const prefix = new Array(len).fill(0).map(() => new Array(2).fill(0));
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  for (let i = 0; i < len; i++) {
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    const idx = s[i] == "1" ? 1 : 0;
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    if (i == 0) prefix[i][idx]++;
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    else {
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      prefix[i] = Array.from(prefix[i - 1]);
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      prefix[i][idx]++;
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    }
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  }
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  let ans = 0;
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  for (let i = 1; i < len - 1; i++) {
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    const c = s[i] == "1" ? 0 : 1;
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    ans += (prefix.at(-1)[c] - prefix[i][c]) * prefix[i - 1][c];
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  }
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  return ans;
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};