Calculate Digit Sum of a String LeetCode solution in JavaScript

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var digitSum = function (s, k) {
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  while (s.length > k) {
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    let newString = "";
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    for (let i = 0; i < s.length; i += k)
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      newString += s
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        .substring(i, i + k)
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        .split("")
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        .reduce((acc, val) => acc + +val, 0);
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    s = newString;
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  }
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  return s;
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};