Issue with credit card checker code

I have an issue with my code, it seems to work but when I test the function with different arrays, the result always shows false, even when I test the function with a valid array. I think my istake is with the algorithm, but i’m not sure. Can someone help explain my mistake? Thanks in advance!

function validateCred(card){
let newCard = card.splice().reverse();
for(let i = 0; i < card.length; i++){
if(i % 2 != 0){
newCard[i] *= 2;
if(newCard[i] > 9){
newCard -= 9);
let total = newCard.reduce((a,b) => a + b, 0);
if(total % 10 === 0){
return true;
} else {
return false;

then it does not seem to work.

if you for an array where you get the wrong result carry it out manually (pen and paper) do you get the right result? and if so, what did you do differently from your code? you can print things out from your code to report what is being done so that you can compare. if things go wrong what do you do? look at what was done, right?