credit_card-checker.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]
const invalid6 = [9, 3, 8, 2, 0, 1, 9, 7, 7, 2, 8, 8, 3, 8, 5, 4] // belongs to an unknown CC company

// 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, invalid5,
    mystery1, mystery2, mystery3, mystery4, mystery5
]

let log = console.log


/**
 * Luhn algorithm: https://en.wikipedia.org/wiki/Luhn_algorithm#Description
 * Visual explanation: https://content.codecademy.com/PRO/independent-practice-projects/credit-card-checker/diagrams/cc%20validator%20diagram%201.svg
 * 
 * @param {array} numbers 
 * @returns {boolean} card status
 */
function validateCred(numbers) {
    // Last element's index of the original array which requires no computation
    const checkDigitIndex = numbers.length - 1

    // The numbers array houses Luhn applied elements
    // It contains the rightmost element which is identical to the last element of original array
    const resultNums = [numbers[checkDigitIndex]]

    // The numbers array will be applied by Luhn algoritm
    const testableNums = numbers.slice(0, checkDigitIndex)

    // Important: this is how to know if doubling a value is required
    const isDoubleRequired = (checkDigitIndex, currentIndex) => (checkDigitIndex - currentIndex) % 2 === 1

    for (let i = testableNums.length - 1; i >= 0; i--) {
        const addingNumber = isDoubleRequired(checkDigitIndex, i) ? testableNums[i] * 2 : testableNums[i];

        // If addingNumber > 9, subtract 9 from its value, otherwise, keep it intact
        resultNums.unshift(addingNumber > 9 ? addingNumber - 9 : addingNumber)
    }

    // sum of resulting numbers mod 10 === 0 => valid
    return resultNums.reduce((prev, curr) => prev + curr) % 10 === 0
}

function findValidCards(cards) {
    return cards.filter(card => validateCred(card));
}

function findInvalidCards(cards) {
    return cards.filter(card => !validateCred(card));
}

/**
 * - Find companies's name which have CC invalid issuing.
 * - Additionally, printout the CC whose numbers belong to unknown companies along the way.
 * 
 * @param {array} cards 
 * @returns {array} companies issuing invalid CC
 */
function idInvalidCardCompanies(cards) {
    const companies = [
        { digit: 3, name: 'Amex (American Express)' },
        { digit: 4, name: 'Visa' },
        { digit: 5, name: 'Mastercard' },
        { digit: 6, name: 'Discover' },
    ]

    const companyDigits = companies.map(company => company.digit)
    const companiesDigitFound = []
    cards.forEach(card => {
        if (companyDigits.includes(card[0]))
            companiesDigitFound.push(card[0])
        else
            console.log(`${card.join('')}: Company not found`)
    })

    const companiesFound = companies.filter(company => companiesDigitFound.includes(company.digit))

    return companiesFound.map(company => company.name);
}

// Testing
log('---< Valid >---\n', findValidCards(batch))
log('---< Invalid >---\n', findInvalidCards(batch))
log('---< Companies with invalid cards >---\n%s', idInvalidCardCompanies([
    invalid1,
    invalid2,
    invalid2,
    invalid2,
    invalid3,
    invalid4,
    invalid5,
    invalid6, // belongs to an unknown CC company
]))

A simple CC checker using the Luhn algorithm.