Apply Discount to Prices LeetCode solution in JavaScript

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var discountPrices = function (sentence, discount) {
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  let isNum = (num) => {
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    if (num.length <= 1 || num[0] != "$") return false;
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    for (let i = 1; i < num.length; ++i)
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      if (!(num[i] >= "0" && num[i] <= "9")) return false;
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    return true;
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  };
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  let x = sentence.split(" ");
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  discount = 1 - discount / 100;
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  for (let i = 0; i < x.length; ++i)
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    !isNum(x[i]) ||
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      (x[i] = `$${(Number(x[i].slice(1)) * discount).toFixed(2)}`);
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  return x.join(" ");
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};