main.js

// 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,
];

// Add your functions below:

const validateCard = (array) => {
  array.reverse();
  let mult = 0;

  for (let index = 0; index < array.length; index++) {
    if (index % 2 === 0) mult += array[index];

    if (index % 2 !== 0) {
      if (array[index] * 2 < 10) {
        mult += array[index] * 2;
      } else {
        mult += array[index] * 2 - 9;
      }
    }
  }

  array.reverse();

  if (mult % 10 === 0) {
    return true;
  } else {
    return false;
  }
};

const findInvalidCards = (array) => {
  let invalidCards = [];

  for (let index = 0; index < array.length; index++) {
    if (validateCard(array[index]) === false) {
      invalidCards.push(array[index]);
    }
  }

  return invalidCards;
};

const idInvalidCardCompanies = (array) => {
  let invalidCardcompanies = [];

  for (let index = 0; index < array.length; index++) {
    const element = array[index];

    if (element[index] === 3) invalidCardcompanies.push("Amex");

    if (element[0] === 4) invalidCardcompanies.push("Visa");

    if (element[0] === 5) invalidCardcompanies.push("MasterCard");

    if (element[0] === 6) {
      invalidCardcompanies.push("Descover");
    } else {
      invalidCardcompanies.push("Company not found");
    }
  }

  invalidCardcompanies = invalidCardcompanies.filter(
    (value, index) => invalidCardcompanies.indexOf(value) === index);

  return invalidCardcompanies

};

console.log(findInvalidCards(batch));
console.log(idInvalidCardCompanies(findInvalidCards(batch)));