freeCodeCamp/curriculum/challenges/espanol/10-coding-interview-prep/rosetta-code/luhn-test-of-credit-card-numbers.md

id, title, challengeType, forumTopicId, dashedName

id	title	challengeType	forumTopicId	dashedName
5ea28156e79528a9ab248f27	Luhn test of credit card numbers	5	385284	luhn-test-of-credit-card-numbers

--description--

The [Luhn test](https://en.wikipedia.org/wiki/Luhn algorithm) is used by some credit card companies to distinguish valid credit card numbers from what could be a random selection of digits.

Those companies using credit card numbers that can be validated by the Luhn test have numbers that pass the following test:

1. Reverse the order of the digits in the number.
2. Take the first, third, ... and every other odd digit in the reversed digits and sum them to form the partial sum s1
3. Taking the second, fourth ... and every other even digit in the reversed digits:
1. Multiply each digit by two and sum the digits if the answer is greater than nine to form partial sums for the even digits.
2. Sum the partial sums of the even digits to form s2.
4. If s1 + s2 ends in zero then the original number is in the form of a valid credit card number as verified by the Luhn test.

For example, if the trial number is 49927398716:

Reverse the digits:
  61789372994
Sum the odd digits:
  6 + 7 + 9 + 7 + 9 + 4 = 42 = s1
The even digits:
    1, 8, 3, 2, 9
  Two times each even digit:
    2, 16, 6, 4, 18
  Sum the digits of each multiplication:
    2, 7, 6, 4, 9
  Sum the last:
    2 + 7 + 6 + 4 + 9 = 28 = s2

s1 + s2 = 70 which ends in zero which means that 49927398716 passes the Luhn test.

--instructions--

Write a function that will validate a number with the Luhn test. Return true if it's a valid number. Otherwise, return false.

--hints--

luhnTest should be a function.

assert(typeof luhnTest === 'function');

luhnTest("4111111111111111") should return a boolean.

assert(typeof luhnTest('4111111111111111') === 'boolean');

luhnTest("4111111111111111") should return true.

assert.equal(luhnTest('4111111111111111'), true);

luhnTest("4111111111111112") should return false.

assert.equal(luhnTest('4111111111111112'), false);

luhnTest("49927398716") should return true.

assert.equal(luhnTest('49927398716'), true);

luhnTest("49927398717") should return false.

assert.equal(luhnTest('49927398717'), false);

luhnTest("1234567812345678") should return false.

assert.equal(luhnTest('1234567812345678'), false);

luhnTest("1234567812345670") should return true.

assert.equal(luhnTest('1234567812345670'), true);

--seed--

--seed-contents--

function luhnTest(str) {

}

--solutions--

function luhnTest(str) {
  var luhnArr = [0, 2, 4, 6, 8, 1, 3, 5, 7, 9];
  var counter = 0;
  var incNum;
  var odd = false;
  var temp = String(str).replace(/[^\d]/g, '');
  if (temp.length == 0) return false;
  for (var i = temp.length - 1; i >= 0; --i) {
    incNum = parseInt(temp.charAt(i), 10);
    counter += (odd = !odd) ? incNum : luhnArr[incNum];
  }
  return counter % 10 == 0;
}