Begginner's attempt, please give me your advice! (credit card checker)

Hello everyone.

I’m a total begginner so please bear with me. I finished this project and I think everything’s working but to be honest I may be missing something (it happened already a few times while I was working on the project). Let me know if you see anything “off” and I look forward to all constructive advice.
Best regards.

pd: Mods, if this post doesn’t belong on this subforum please let me know. Thanks!

//FUNCTION TO VALIDATE CARDS

function validateCred(cards) {

let cardsReverse = cards.slice().reverse();

for (i=0; i < cardsReverse.length; i++ ) {

if (i % 2 != 0) {

  cardsReverse[i] *= 2;

};

if (cardsReverse[i] > 9) {

  cardsReverse[i] -= 9;

};

};

let cardsSum = cardsReverse.reduce((a, b) => a + b);

if (cardsSum % 10 === 0) {

return true;

} else {

return false;

};

};

//FUNCTION TO FIND INVALID CARDS

const findInvalidCards = (cards) =>{

let invalidCard = ;

for (let j = 0; j < cards.length; j++) {

if(validateCred(cards[j]) === false) {

  invalidCard.push(cards[j]);

  

}

}

return invalidCard;

};

//FUNCTION TO DETECT WHICH COMPANIES ARE INVALID

function idInvalidCardCompanies(invCardArr) {

let companies = ;

let cards = invCardArr.map((item) => {

if (validateCred(item) === false) {

  if (item[0] === 3) {

    companies.push('amex');

  } else if (item[0] === 4) {

    companies.push('visa');

  } else if (item[0] === 5) {

    companies.push('mastercard');

  } else if (item[0] === 6) {

    companies.push('discover');

  } else {

    companies.push(`card with number ${item} belongs to an unknown company`);

}

}

});

compNoDupli = companies.filter((item, pos) => {

return companies.indexOf(item) == pos;

});

return compNoDupli;

};

//FUNCTION TO CONVERT CC STRINGS/NUMBERS TO ARRAYS

function convertCc(input) {

newArr = Array.from(String(input), Number);

return newArr;

};

//FUNCTION TO CONVERT INVALID CARD TO VALID

let invalidToValid = (cards) => {

let cardsReverse = cards.slice().reverse();

for (i=0; i < cardsReverse.length; i++ ) {

if (i % 2 != 0) {

  cardsReverse[i] *= 2;

};

if (cardsReverse[i] > 9) {

  cardsReverse[i] -= 9;

};

};

let cardsSum = cardsReverse.reduce((a, b) => a + b);

let modulo = cardsSum % 10;

for (i=0; i < cards.length; i++) {

if (i % 2 != 0 && i > modulo) {

  cards.splice([i], 1, cards[i] - modulo)

  break;

  };

};

return cards;

};