Credit Card Checker - need advice from Pro

Hey everyone!

I just started to learn programming at 36 (with no prior experience) and finally managed to finish this project (took almost 1 whole day), however, I feel my coding is too complex, sluggish and not efficient. I feel as if I lack some math or algorithms knowledge and sometimes feel really sad about that. What I mean by 1 whole day is not just plain coding but walking in my flat, looking into the window thoughtfully and finally trying to concentrate and find a proper solution =) At the end - I do like the feeling of solving a problem which I really admire.

Can someone give me advice if that’s ok and I will catch up or should I start learn something else besides coding to improve my coding structure, approach, etc.

Also, question on my code: it was told not to mutate array in function however I did mutate it at start and restored to its original state at the end of function, is this a normal practice?

function validateCred(cardArray) {
let cardArrayCopy = cardArray; // evade mutating original array
let cardArraySum = cardArrayCopy.pop();
let unMutate = cardArraySum;

for (i = cardArrayCopy.length - 1; i >= 0; i -= 2) {
//odd num check
if (cardArrayCopy[i] * 2 > 9) {
cardArraySum += cardArrayCopy[i] * 2 - 9;
} else {
cardArraySum += cardArrayCopy[i] * 2;
}
}

for (j = cardArrayCopy.length - 2; j >= 0; j -= 2) {
//even num check
cardArraySum += cardArrayCopy[j];
}

cardArrayCopy.push(unMutate); // restore original array

if (cardArraySum % 10 === 0) {
return true;
} else {
return false;
}
}

// All valid credit card numbers
const valid1 = [4, 5, 3, 9, 6, 7, 7, 9, 0, 8, 0, 1, 6, 8, 0, 8];
const valid2 = [5, 5, 3, 5, 7, 6, 6, 7, 6, 8, 7, 5, 1, 4, 3, 9];
const valid3 = [3, 7, 1, 6, 1, 2, 0, 1, 9, 9, 8, 5, 2, 3, 6];
const valid4 = [6, 0, 1, 1, 1, 4, 4, 3, 4, 0, 6, 8, 2, 9, 0, 5];
const valid5 = [4, 5, 3, 9, 4, 0, 4, 9, 6, 7, 8, 6, 9, 6, 6, 6];

// All invalid credit card numbers
const invalid1 = [4, 5, 3, 2, 7, 7, 8, 7, 7, 1, 0, 9, 1, 7, 9, 5];
const invalid2 = [5, 7, 9, 5, 5, 9, 3, 3, 9, 2, 1, 3, 4, 6, 4, 3];
const invalid3 = [3, 7, 5, 7, 9, 6, 0, 8, 4, 4, 5, 9, 9, 1, 4];
const invalid4 = [6, 0, 1, 1, 1, 2, 7, 9, 6, 1, 7, 7, 7, 9, 3, 5];
const invalid5 = [5, 3, 8, 2, 0, 1, 9, 7, 7, 2, 8, 8, 3, 8, 5, 4];

// Can be either valid or invalid
const mystery1 = [3, 4, 4, 8, 0, 1, 9, 6, 8, 3, 0, 5, 4, 1, 4];
const mystery2 = [5, 4, 6, 6, 1, 0, 0, 8, 6, 1, 6, 2, 0, 2, 3, 9];
const mystery3 = [6, 0, 1, 1, 3, 7, 7, 0, 2, 0, 9, 6, 2, 6, 5, 6, 2, 0, 3];
const mystery4 = [4, 9, 2, 9, 8, 7, 7, 1, 6, 9, 2, 1, 7, 0, 9, 3];
const mystery5 = [4, 9, 1, 3, 5, 4, 0, 4, 6, 3, 0, 7, 2, 5, 2, 3];

// An array of all the arrays above
const batch = [valid1, valid2, valid3, valid4, valid5, invalid1, invalid2, invalid3, invalid4, invalid5, mystery1, mystery2, mystery3, mystery4, mystery5];

// Add your functions below:

function validateCred(cardArray) {
  let cardArrayCopy = cardArray; // evade mutating original array
  let cardArraySum = cardArrayCopy.pop();
  let unMutate = cardArraySum;
  for (i = cardArrayCopy.length - 1; i >= 0; i -= 2) {
    //odd num check
    if (cardArrayCopy[i] * 2 > 9) {
      cardArraySum += cardArrayCopy[i] * 2 - 9;
    } else {
      cardArraySum += cardArrayCopy[i] * 2;
    }
  }
  for (j = cardArrayCopy.length - 2; j >= 0; j -= 2) {
    //even num check
    cardArraySum += cardArrayCopy[j];
  }

  cardArrayCopy.push(unMutate); // restore original array

  if (cardArraySum % 10 === 0) {
    return true;
  } else {
    return false;
  }
}

function findInvalidCards(cardsArray) {
  let invalidCards = [];
  for (let i = 0; i < cardsArray.length; i++) {
    let singleCard = cardsArray[i];
    if (validateCred(singleCard) === false) {
      invalidCards.push(singleCard);
    }
  }
  return invalidCards;
}

function idInvalidCardCompanies(allCards) {
  let invalidCompanyCards = [];
  let invalid = findInvalidCards(allCards); // getting invalid cards again

  for (let i = 0; i < invalid.length; i++) {
    let singleCard = invalid[i];

    switch (singleCard[0]) {
      case 3:
        invalidCompanyCards.push('Amex ' + singleCard);
        break;
      case 4:
        invalidCompanyCards.push('Visa ' + singleCard);
        break;
      case 5:
        invalidCompanyCards.push('Master Card ' + singleCard);
        break;
      default:
        invalidCompanyCards.push('Unknown ' + singleCard);
        break;
    }
  }
  return invalidCompanyCards;
}

console.log(validateCred(valid1)); // test valid card
console.log(validateCred(invalid1)); // test invalid card
console.log(findInvalidCards(batch)); // test invalid card multiple numbers
console.log(idInvalidCardCompanies(batch)); // test card companies (invalid only)