Ajax doesn’t get a success response back after file upload

I just started with Ajax and also tried to find a solution for this.

Here is the problem:

I upload a .csv to a server. This works just fine. But after the upload “success” in the ajax call won’t respond. Neither does complete or error. It shows nothing. Not even an empty alert. Just nothing. I also didn’t find anything in the logs.

When I click the upload button without chosing a file to upload I get a response from success…

Here is the code:

upload.php

<?php
header('Content-type: application/json');
$target_dir = 'uploads/';
$target_file = $target_dir . basename($_FILES["file"]["name"]);
$response[] ='';

  if(move_uploaded_file($_FILES["file"]["tmp_name"],$target_file))
  { 
    //$response['message'] = "File was uploaded!";
    $response['message'] = csvToJson($target_file);
  }
  else
  { 
    $response['message'] = 'Sorry, there was an error uploading your file.'; 
  }

echo json_encode($response);


function csvToJson($file) 
{
    // open csv file
    if (!($fp = fopen($file, 'r'))) {
        die("Can't open file...");
    }
    
    //read csv headers
    $key = fgetcsv($fp,"1024",";");
    
    // parse csv rows into array
    $json = array();
        while ($row = fgetcsv($fp,"1024",";")) {
        $json[] = array_combine($key, $row);
    }
    
    // release file handle
    fclose($fp);
    
    // encode array to json
    return $json;
}
?>

upload.js

$(document).ready(function(e)
{
    // Submit form data via Ajax
    $("#form").on('submit', function(e)
    {
        var form_data = new FormData($(this)[0]);  
        $.ajax({    
            type: 'POST',
            url: 'upload.php',
            data: form_data,
            contentType: false,
            cache: false,
            processData:false,
            success: function(response)
            { 
                //var json = $.parseJSON(response)
                alert(response);
            },
            error: function(error){
                console.log("Error:");
                console.log(error);
           }, 
           complete: function(response){
            alert(response);
           }

        });
    });
});

the form in index.php

<form id=form enctype="multipart/form-data">
        <input type="file" name="file" id="file">
        <input type="submit" value="Upload Excel" name="submit">
</form>

In the end I want a json back with the content of the uploaded file. But right now there is zero output if the file is uploaded.

Thanks for any advice!