Credit Card Checker Project I completed on Codecademy. This application takes in credit card numbers and validates them using the Luhn algorithm.

creditCardChecker.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];


//Converts string arrays into number arrays
const convertArray = array => {
  let newArray = [];
  for (var i = 0; i < array.length; i++ ){
    newArray.push(Number(array[i]));
  }
  return newArray
}

//Validates credit cards using the Luhn algorithm.
//General steps of this algorithm are: 
/*
  1.Starting from the farthest digit to the right, AKA the check digit, iterate to the left.
  2. As you iterate to the left, every other digit is doubled (the check digit is not doubled). If the number is greater than 9 after doubling, subtract 9 from its value.
  3. Sum up all the digits in the credit card number.
  4. If the sum modulo 10 is 0 (if the sum divided by 10 has a remainder of 0) then the number is valid, otherwise, it’s invalid.
*/

  const validateCred = newArray => {
    newArray.reverse();
    let sumNumbers = 0;
  for (var i = 0; i < newArray.length; i++ ){
if (i % 2 == 0 ){
  sumNumbers += newArray[i];
} else{
  sumNumbers += calculateValues(newArray[i]);
}
  }
  newArray.reverse()
 if (sumNumbers % 10 === 0){
   return true;
 }
 else {
   return false;
 }
};
// Helper function for validateCred
const calculateValues = numberToBeChecked => {
let result = undefined;
result = numberToBeChecked * 2;
if (result > 9){
  result = result - 9;
  return result;
} else {
  return result;
}
}


const findInvalidCards = array => {
  let invalidArrays = [];
  for (j = 0; j < array.length; j++){
    if (validateCred(array[j]) === false){
    invalidArrays.push(array[j]);
    }
  }
return invalidArrays; 
};

//Checks the first number of the invalid credit card numbers and returns the corresponding company. Also removes duplicates so that the company is only return once.
const idInvalidCompanies = array => {
  let invalidatedCompanies = [];
  for (i = 0; i < array.length; i++){
    switch (array[i][0]){
      case 3 :
      invalidatedCompanies.push('Amex');
      break;
      case 4 :
      invalidatedCompanies.push('Visa');
      break;
      case 5 :
      invalidatedCompanies.push('Mastercard');
      break;
      case 6 :
      invalidatedCompanies.push('Discover');
      break;
      default:
      invalidatedCompanies.push('Company not found');
    }
  }
  //Removes duplicates, returning company one time only.
  function removeDuplicates(array){
    let unique = [];
    array.forEach(element => {
      if (!unique.includes(element) ){
        unique.push(element);
      }
    });
    return unique;
  }
  return console.log(removeDuplicates(invalidatedCompanies));
  
}
idInvalidCompanies(findInvalidCards(batch));
//Checks to see if function returns array of invalid cards: console.log(findInvalidCards(batch))
//Checks to see if function validates credit card: console.log(validateCred(invalid1))