Masking Personal Information LeetCode solution in JavaScript

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var maskPII = function (S) {
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  if (isEmail(S)) return maskEmail(S);
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  return maskPhone(S);
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  function isEmail(str) {
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    return /^[A-Za-z]{2,}@[A-Za-z]{2,}.[A-Za-z]{2,}$/.test(str);
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  }
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  function maskEmail(str) {
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    let res = "";
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    const [local, domain] = str.split("@");
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    res +=
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      local.charAt(0).toLowerCase() +
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      "*****" +
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      local.charAt(local.length - 1).toLowerCase();
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    return res + "@" + domain.toLowerCase();
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  }
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  function maskPhone(str) {
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    const onlyDigits = str.replace(/\D/g, "");
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    const localNum = "***-***-" + onlyDigits.substring(onlyDigits.length - 4);
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    if (onlyDigits.length === 10) return localNum;
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    let countryNum = "+" + "*".repeat(onlyDigits.length - 10) + "-";
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    return countryNum + localNum;
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  }
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};