Why is my JavaScript Fetch getting an error on successful post on my Laravel App

I have a Laravel app where my blade view makes a JavaScript fetch post to a controller method. I don't understand why my fetch would catch an error on some successful post. My fetch is synchronous.

I had previously used Ajax and got the same results.

Here are the codes. On my blade view, this is my JavaScript code

<script type="text/javascript">
async function fetchPostCards(url, data_post){
  return await fetch(url, {
    method: 'POST', // or 'PUT'
    body: JSON.stringify(data_post), // data can be `string` or {object}!
    headers:{
      'Content-Type': 'application/json'
    }
  })
  .then(response => response.json());
}

function processCards(){
  //this is called via button click
  let arrCard = [
    {card_number: "ABC001", email: "johndoe@somewhere.com"},
    {card_number: "ABC002", email: "janedoe@somewhere.com"},
    {card_number: "ABC003", email: "jimdoe@somewhere.com"},
    {card_number: "ABC004", email: "jackdoe@somewhere.com"},
    {card_number: "ABC005", email: "janetdoe@somewhere.com"},
  ];
  let csrf_token = $( "input[name*='_token']" ).val();

  arrCard.forEach(function(indCard){
    let post_data = {};
    post_data.card_number = indCard.card_number;
    post_data.email = indCard.email;
    data_post['_token'] = csrf_token;

    var fetchPost = fetchPostCards('/processcards', data_post)
    .then(data => {
      console.log(JSON.stringify(data));
    })
    .catch(error => {
      console.log("error " + error);             

    });

  });
}
</script>

Here is my code on the controller (PHP):

use Log;

class CardController extends Controller
{

  public function processcard(Request $request){
    $card_number = $request->card_number;
    $email = $request->email;

    $result = "";

    if($card_number == "ABC004") $nonVar += 4; #simulate an error

    #do processing here in the cards
    #some will have $result = "SUCCESS"
    #some will have $result = "CARD NOT VALID", etc.

    log::info("card number: ".$card_number);
    log::info("activate_status: ".$activate_status);
    log::info("******");
    return response()->json(['card_number' =>  $card_number, 'result' => $result]);
  }

}

On request for "ABC004", the fetch catches the error, which I expected. On result = "SUCCESS", the fetch catches the error. But when I checked the Laravel logs, I see the "SUCCESS" results with no error.

How do I fix this?

via Chebli Mohamed