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/**

* Validate a given credit card number

*

* The core of the validation of credit card numbers is the Luhn algorithm.

*

* The validation sum should be completely divisible by 10 which is calculated as follows,

* every first digit is added directly to the validation sum.

* For every second digit in the credit card number, the digit is multiplied by 2.

* If the product is greater than 10 the digits of the product are added.

* This resultant digit is considered for the validation sum rather than the digit itself.

*

* Ref: https://www.geeksforgeeks.org/luhn-algorithm/

*/

const luhnValidation = (creditCardNumber) => {

let validationSum = 0

creditCardNumber.split('').forEach((digit, index) => {

let currentDigit = parseInt(digit)

if (index % 2 === 0) {

// Multiply every 2nd digit from the left by 2

currentDigit *= 2

// if product is greater than 10 add the individual digits of the product to get a single digit

if (currentDigit > 9) {

currentDigit %= 10

currentDigit += 1

}

}

validationSum += currentDigit

})

return validationSum % 10 === 0

}

const validateCreditCard = (creditCardString) => {

const validStartSubString = ['4', '5', '6', '37', '34', '35'] // Valid credit card numbers start with these numbers

if (typeof creditCardString !== 'string') {

throw new TypeError('The given value is not a string')

}

const errorMessage = `${creditCardString} is an invalid credit card number because `

if (isNaN(creditCardString)) {

throw new TypeError(errorMessage + 'it has nonnumerical characters.')

}

const creditCardStringLength = creditCardString.length

if (!((creditCardStringLength >= 13) && (creditCardStringLength <= 16))) {

throw new Error(errorMessage + 'of its length.')

}

if (!validStartSubString.some(subString => creditCardString.startsWith(subString))) {

throw new Error(errorMessage + 'of its first two digits.')

}

if (!luhnValidation(creditCardString)) {

throw new Error(errorMessage + 'it fails the Luhn check.')

}

return true

}

export { validateCreditCard }