Codecademy export

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:
function validateCred(card) {
  let creditCard = [];
  let sumDigits = 0;
  let digit = 0;
  for (let item of card) { creditCard.unshift(item); }
  for (let i = 0; i < creditCard.length; i += 1) {
    if (i % 2 == 0) {
      digit = creditCard[i];
    } else if (creditCard[i] * 2 > 9) {
      digit = (creditCard[i] * 2) - 9;
    } else {
      digit = creditCard[i] * 2;
    }
    sumDigits += digit;
  }
  return sumDigits % 10 == 0;
}

//4 
function findInvalidCards(cards) {
  return cards.filter((item) => !validateCred(item));
}

//5
function idInvalidCardCompanies(invalidCards) {
  let invalidCompanies = [];
  for (let item of invalidCards) {
    switch (item[0]) {
      case 3:
        if (invalidCompanies.indexOf('Amex') === -1) { 
          invalidCompanies.push('Amex');
         }
        break;
      case 4:
        if (invalidCompanies.indexOf('Visa') === -1) { 
          invalidCompanies.push('Visa');
         }
        break;
      case 5:
        if (invalidCompanies.indexOf('Mastercard') === -1) { 
          invalidCompanies.push('Mastercard');
        }
        break;
      case 6:
        if (invalidCompanies.indexOf('Discover') === -1) { 
          invalidCompanies.push('Discover');
         }
        break;
    }
  }
  return invalidCompanies;
}

very cool taught process, like it but you could improve your code a little and
optimize it and it'll be great if could get rid of code duplication: ie
for (let i = 0; i < creditCard.length; i += 1) {
if (i % 2 == 0) {
digit = creditCard[i];
} else if (creditCard[i] * 2 > 9) {
digit = (creditCard[i] * 2) - 9;
} else {
digit = creditCard[i] * 2;
}
sumDigits += digit;
}
return sumDigits % 10 == 0;
}