Count Number of Homogenous Substrings LeetCode solution in JavaScript

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var countHomogenous = function (s) {
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  let mod = 1e9 + 7;
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  let n = s.length;
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  let j = 0,
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    res = 0;
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  for (let i = 0; i < n; i++) {
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    if (i > 0 && s[i - 1] != s[i]) {
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      let x = i - j;
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      res += (x * (x + 1)) / 2;
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      j = i;
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    }
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  }
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  let x = n - j;
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  res += (x * (x + 1)) / 2;
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  return res % mod;
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};