freecodecamp--freecodecamp/curriculum/challenges/english/blocks/daily-coding-challenges-javascript/6a0dcd03ee4e68698080ef6c.md

id, title, challengeType, dashedName

id	title	challengeType	dashedName
6a0dcd03ee4e68698080ef6c	Challenge 308: Credit Card Validator	28	challenge-308

--description--

Given a string of digits for a credit card number, determine if it's a valid card number using the following method:

• Starting from the second-to-last digit, double every other digit moving left.
• If doubling a digit results in a number greater than 9, subtract 9.
• Sum all the digits (doubled and undoubled).
• If the total is divisible by 10, the number is valid.

--hints--

isValidCard("4532015112830366") should return true.

assert.isTrue(isValidCard("4532015112830366"));

isValidCard("5425233430109903") should return true.

assert.isTrue(isValidCard("5425233430109903"));

isValidCard("371449635398431") should return true.

assert.isTrue(isValidCard("371449635398431"));

isValidCard("6011111111111117") should return true.

assert.isTrue(isValidCard("6011111111111117"));

isValidCard("4532015112830367") should return false.

assert.isFalse(isValidCard("4532015112830367"));

isValidCard("1234567890123456") should return false.

assert.isFalse(isValidCard("1234567890123456"));

isValidCard("4532015112830368") should return false.

assert.isFalse(isValidCard("4532015112830368"));

--seed--

--seed-contents--

function isValidCard(number) {

  return number;
}

--solutions--

function isValidCard(number) {
  let sum = 0;

  for (let i = number.length - 1; i >= 0; i--) {
    let digit = parseInt(number[i]);

    if ((number.length - 1 - i) % 2 === 1) {
      digit *= 2;
      if (digit > 9) digit -= 9;
    }

    sum += digit;
  }

  return sum % 10 === 0;
}