How can I get next check digit using luhn algorithm in javascript

Question

I have the following code that validates if a certain digits is valid using luhn algorithm module 10.

function isCheckdigitCorrect(value) {
// accept only digits, dashes or spaces
  if (/[^0-9-\s]+/.test(value)) return false;

  var nCheck = 0, nDigit = 0, bEven = false;
  value = value.replace(/\D/g, "");

  for (var n = value.length - 1; n >= 0; n--) {
    var cDigit = value.charAt(n),
      nDigit = parseInt(cDigit, 10);

    if (bEven) {
      if ((nDigit *= 2) > 9) nDigit -= 9;
    }

    nCheck += nDigit;
    bEven = !bEven;
  }

  return (nCheck % 10) == 0;
}

I need another function that generates the next check-digit actually by giving four digit number so the 5th digit would be next digit checksum.

_"I need another function"_ I understand, but can you post what have you tried so far? — evolutionxbox, Apr 24 '17 at 11:52
@evolutionxbox i havent tried anything yet cz I have no idea how to achieve that, actually i tried the same function and modified a litle bit but had no luck with it! — pranvera hoti, Apr 24 '17 at 11:55
@M98 this is what I already have actually, this function checks if the digits are valid based on luhn algorithm, I need to create digits including the checksum — pranvera hoti, Apr 24 '17 at 11:56
@M98 Ohh I see the comments below are telling how to generate one, worked, thnx :) — pranvera hoti, Apr 24 '17 at 11:57
@pranverahoti You're welcome, Try this one instead: https://gist.github.com/ShirtlessKirk/2134376 — Mohammad Kermani, Apr 24 '17 at 12:03
@pranverahoti Did you know you can answer your own question on Stack Overflow? Go on, answer this question yourself. It might be helpful for someone who found this question by Google. When you answered that, let me know — Mohammad Kermani, Apr 24 '17 at 12:05

score 3 · Accepted Answer · answered Apr 24 '17 at 12:31

By modifying the current function to this one I was able to get next checkdigit:

function getCheckDigit(value) {
  if (/[^0-9-\s]+/.test(value)) return false;

  var nCheck = 0, nDigit = 0, bEven = true;
  value = value.replace(/\D/g, "");

  for (var n = value.length - 1; n >= 0; n--) {
    var cDigit = value.charAt(n),
      nDigit = parseInt(cDigit, 10);

    if (bEven) {
      if ((nDigit *= 2) > 9) nDigit -= 9;
    }

    nCheck += nDigit;
    bEven = !bEven;
  }
  return (1000 - nCheck) % 10;
}

Perfecto! Good to see you answered a question for the first time, you can also accept your answer as the correct answer. Stack Overflow is an open community, it's why I like it. +1 for you — Mohammad Kermani, Apr 24 '17 at 12:47

How can I get next check digit using luhn algorithm in javascript

1 Answers1