fiscalCode.js

const fiscalCode = (person) => {
  // Inital setup

  let { name: firstName, surname: lastName, gender, dob } = person;
  const months = { 1: "A", 2: "B", 3: "C", 4: "D", 5: "E", 6: "H", 7: "L", 8: "M", 9: "P", 10: "R", 11: "S", 12: "T" };
  dob = person.dob.split("/").map((n) => +n);

  // Build parts

  function getLetters(string, letterType = "consonants") {
    let vowels = string.match(/[aeiou]/gi);
    let consonants = string.match(/[^aeiou]/gi);

    if (letterType !== "consonants") {
      return vowels;
    }

    return consonants;
  }

  function getLastNamePart(num) {
    let part;
    let lettersArr = getLetters(lastName);

    if (lastName.length < 3) {
      part = [...lastName].concat(["x"]);
    } else if (num >= 3) {
      part = lettersArr.slice(0, 3);
    } else if (num < 3) {
      part = lettersArr.slice(0, 2).concat(getLetters(lastName, "vowels").slice(0, 1));
    } else {
      part = null;
    }

    return part;
  }

  function getFirstnamePart(num) {
    let part;
    let lettersArr = getLetters(firstName);

    if (firstName.length < 3) {
      part = [...firstName].reverse().concat(["x"]);
    } else if (num > 3) {
      part = [lettersArr[0] + lettersArr[2] + lettersArr[3]];
    } else if (num === 3) {
      part = lettersArr;
    } else if (num < 3) {
      part = lettersArr.slice(0, 2).concat(getLetters(firstName, "vowels").slice(0, 1));
    } else {
      part = null;
    }

    return part;
  }

  function getGenderAndDobPart(letter, date) {
    let part;
    let [day, month, year] = date;

    year = String(year).slice(2);
    month = months[month];

    if (letter === "F") {
      day += 40;
    } else if (letter === "M" && day < 10) {
      day = 0 + String(day);
    } else {
      day = day;
    }

    part = [year, month, day];

    return part;
  }

  // Build the final result

  let consonantsInFirstName = getLetters(firstName).length;
  let consonantsInLastName = getLetters(lastName).length;

  let lastNamePart = getLastNamePart(consonantsInLastName);
  let firstNamePart = getFirstnamePart(consonantsInFirstName);
  let genderAndDobPart = getGenderAndDobPart(gender, dob);

  let parts = [lastNamePart, firstNamePart, genderAndDobPart];
  let final = parts
    .map((part) => part.join(""))
    .join("")
    .toUpperCase();

  return final;
};