Decoded String at Index LeetCode solution in JavaScript

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var decodeAtIndex = function (s, k) {
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  let len = 0;
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  let isDigit = false;
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  for (let v of s) {
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    if (v >= "0" && v <= "9") {
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      len *= +v;
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      isDigit = true;
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    } else {
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      len++;
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      if (len === k && !isDigit) {
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        return s[k - 1];
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      }
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    }
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  }
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  for (let i = s.length - 1; i >= 0; i--) {
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    const v = s[i];
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    if (v >= "0" && v <= "9") {
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      len = Math.ceil(len / +v); // Math.floor() wont work because we endup leaving few strings
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      k %= len;
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    } else {
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      if (k === 0 || k === len) {
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        return v;
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      }
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      len--;
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    }
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  }
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};