Credit Card Checker Solution

A solution for Credit Card Checker. Does not include the project extensions.

// All valid credit card numbers
const valid1 = [4, 5, 3, 9, 6, 7, 7, 9, 0, 8, 0, 1, 6, 8, 0, 8];
const valid2 = [5, 5, 3, 5, 7, 6, 6, 7, 6, 8, 7, 5, 1, 4, 3, 9];
const valid3 = [3, 7, 1, 6, 1, 2, 0, 1, 9, 9, 8, 5, 2, 3, 6];
const valid4 = [6, 0, 1, 1, 1, 4, 4, 3, 4, 0, 6, 8, 2, 9, 0, 5];
const valid5 = [4, 5, 3, 9, 4, 0, 4, 9, 6, 7, 8, 6, 9, 6, 6, 6];

// All invalid credit card numbers
const invalid1 = [4, 5, 3, 2, 7, 7, 8, 7, 7, 1, 0, 9, 1, 7, 9, 5];
const invalid2 = [5, 7, 9, 5, 5, 9, 3, 3, 9, 2, 1, 3, 4, 6, 4, 3];
const invalid3 = [3, 7, 5, 7, 9, 6, 0, 8, 4, 4, 5, 9, 9, 1, 4];
const invalid4 = [6, 0, 1, 1, 1, 2, 7, 9, 6, 1, 7, 7, 7, 9, 3, 5];
const invalid5 = [5, 3, 8, 2, 0, 1, 9, 7, 7, 2, 8, 8, 3, 8, 5, 4];

// Can be either valid or invalid
const mystery1 = [3, 4, 4, 8, 0, 1, 9, 6, 8, 3, 0, 5, 4, 1, 4];
const mystery2 = [5, 4, 6, 6, 1, 0, 0, 8, 6, 1, 6, 2, 0, 2, 3, 9];
const mystery3 = [6, 0, 1, 1, 3, 7, 7, 0, 2, 0, 9, 6, 2, 6, 5, 6, 2, 0, 3];
const mystery4 = [4, 9, 2, 9, 8, 7, 7, 1, 6, 9, 2, 1, 7, 0, 9, 3];
const mystery5 = [4, 9, 1, 3, 5, 4, 0, 4, 6, 3, 0, 7, 2, 5, 2, 3];

// An array of all the arrays above
const batch = [valid1, valid2, valid3, valid4, valid5, invalid1, invalid2, invalid3, invalid4, invalid5, mystery1, mystery2, mystery3, mystery4, mystery5];

// validates the card assuming it's an array of numbers
const validateCred = (card) => {
  // inverse card array used for validation
  const inverseCard = [];
  for (var num of card) {
    inverseCard.unshift(num);
  }

  // calculates sum with Luhn algorithm
  let sum = 0;
  // starting from one index past the check digit
  for (let i = 1; i < inverseCard.length; i++) {
    // the digits at odd indexes are doubled
    if (i % 2 === 1) {
      if (inverseCard[i] * 2 > 9) {
        sum += inverseCard[i] * 2 - 9;
      } else {
        sum += inverseCard[i] * 2;
      }
    } else {
      sum += inverseCard[i];
    }
  }
  // end of sum

  // returns the comparison between the given check digit and the calculated check digit
  return card[card.length -1] === 10 - (sum % 10);
};

// negated version of filter array method
const findInvalidCards = (batch) => {
  const invalidCards = [];
  for (var card of batch) {
    if (!validateCred(card)) {
      invalidCards.push(card);
    }
  }
  return invalidCards;
};

// returns the first occurrence of the company with companyDigit
const findCompany = (invalidCards, companyDigit) => {
  // the card's check digit is compared against the companyDigit parameter
  return invalidCards.find(card => card[card.length - 1] === companyDigit);
};

// searches invalidCards for known companies
const idInvalidCardCompanies = (invalidCards) => {
  const companies = [];
  if (findCompany(invalidCards, 3)) {
    companies.push("AMEX");
  }
  if (findCompany(invalidCards, 4)) {
    companies.push("VISA");
  }
  if (findCompany(invalidCards, 5)) {
    companies.push("MasterCard");
  }
  if (findCompany(invalidCards, 6)) {
    companies.push("Discover");
  }
  // end of company search

  // returns an array of companies
  // otherwise, undefined if no company was found
  if (companies.length === 0) {
    console.log("Company not found");
    return;
  }
  return companies;
};

// test
for (var card of batch) {
  console.log(validateCred(card)); // 5 times true, 6 times false, true, 2 times false, true
}
const invalidCards = findInvalidCards(batch);
console.log(invalidCards); // array of 8 invalid card arrays
console.log(idInvalidCardCompanies(invalidCards)); // expected [ 'AMEX', 'VISA', 'MasterCard' ]

The algorithm takes after the Wikipedia page.