Day42.js

//* Programming Challenge: Validate a Credit Card Number (Luhn Algorithm)

//? Write a function to validate credit card numbers using the Luhn algorithm.
//* Requirements:
//? Write a function named validateCreditCard that takes a credit card number as a string.
//? The function should return true if the number is valid according to the Luhn algorithm, and false otherwise.
//? Ensure the function can handle numbers as strings, which may include spaces and dashes.
//* Luhn Algorithm Steps:
// Steps of the Luhn Algorithm
// Prepare the Number:
// Start with the digits of the number. For example, if validating the number 79927398713.
// I Reverse the Digits:
// Reverse the digits of the number. For the example, it becomes 31789379297.
// I Double Every Second Digit:
// Starting from the first digit, double every second digit.
// For our example: 3, 1*2, 7, 8*2, 9, 3*2, 7, 9*2, 2, 7*2, 9.
// Which translates to: 3, 2, 7, 16, 9, 6, 7, 18, 2, 14, 9.
// substract  9 from  number  higher than 9 ****OR****  If doubling the number results in a number greater than 9, subtract 9 from it.

// Now our series is: 3, 2, 7, 7, 9, 6, 7, 9, 2, 5, 9.
// Sum All Digits:
// Add all the digits together.
// 113+2+7+7+9+6+7+9+2+5 + 9= 66.
// Check Modulo 10:
// If the sum modulo 10 is 0, then the number is valid according to the Luhn formula.
// 66 % 10 = 6, which is not 0, so 79927398713 is not a valid number according to Luhn.

const validateCreditCard = (str) => {
  // Reverse the Digits:
  //   let  reverse =   str.split("").reverse().join("");
  //     console.log(reverse);
  str = str.replace(/\D/g, "");
  let revNum = "";
  for (let i = str.length - 1; i >= 0; i--) {
    revNum = revNum + str[i];
  }
  console.log(revNum);
  // Double Every Second Digits
  // map is not used in string so we used split() method
  let doubleNum = revNum
    .split("")
    .map((currDigits, index) => {
      if (index % 2 !== 0) {
        currDigits = currDigits * 2;

        if (currDigits > 9) {
          // substract  9 from  number  higher than 9
          currDigits = currDigits - 9;
        } else {
          currDigits = currDigits;
        }
      }
      return currDigits;
    })
    .reduce((accum, currElem) => accum + Number(currElem), 0); //  Add all the digits together.
    console.log(doubleNum);
  return doubleNum % 10 === 0;
};
console.log(validateCreditCard("4539 1488 0343 6467")); // Output: true
console.log(validateCreditCard("8273 1232 7352 0569")); // Output: false