Credit Card Checker: Array Modification and Definition Issue

Hi! In the Credit Card checker task I stumbled upon an issue with modifying arrays. Below in the code I have modCardNumber array that I defined as equal to cardNumber. I would then modify modCardNumber with the loops. In the end to check, I print both arrays and see how modCardNumber has changed. However, cardNumber is now the same as modCardNumber. Why is that happening? I think, I haven’t fully understood variable definitions in prior lessons, but not sure.

// All valid credit card numbers
const valid1 = [4, 5, 3, 9, 6, 7, 7, 9, 0, 8, 0, 1, 6, 8, 0, 8];

// Add your functions below:
function validateCred(cardNumber) {
  let modCardNumber = cardNumber;
  for (let i = modCardNumber.length-2; i >= 0; i -= 2) {
      modCardNumber[i] *= 2;
      if (modCardNumber[i] > 9) {
        modCardNumber[i] -= 9;


i am not sure this issue has been covered at this point, but this line:

let modCardNumber = cardNumber;

works different then you expect.

Seems you got pretty far figuring out what the problem is:

that is because both variable point to the same array in memory. So you only have one array. Here is a simple example to help you grasp it:

const one = ['a', 'b', 'c'];
const two = one;

console.log(one, two);
one[0] = 'd';
console.log(one, two);
two[1] = 'e';
console.log(one, two);

anyway, here:

let modCardNumber = cardNumber;

you need to make sure you copy the array.

you could practice this with the simple example i provided

just for myself, the project url:, so i can find the project more easily next time, might i need to help you further

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Hi stetim94! Thank you very much for the thorough explanation. The simple phrase of ‘cope the array’ guided me directly to:

let modCardNumber = cardNumber.slice();

I am not sure that this is the best way to do it, but seems to solve the problem you described.

Thanks again!

you’re welcome :slight_smile:

given you use es6, i would use the spread operator:

let modCardNumber = [...cardNumber];

.slice() seemed to be the way to go before es6.

the spread operator (at least to me) is clearer in the fact that you create a new array.

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