I can\'t understand why I get always a null or undefined response while trying to pass an php error from registration form into ajax script. I work in a custom plugin to add login, registration and dashboard with the folder.

History : I create an ajax registration form , for each field I want create an error (if there are errors) and display it below the field.

I tried with basic var ($_POST), json_encode and wp_send_json_error. Each time I get a null or undefined response in console.log.

I know that my ajax script work because data $_POST are well pass. But I can\'t solve my issue or I don\'t understand what I do with Json !

I have read ajax doc and followed lot of docs about the same issue without results.

Currently basic thing doesn\'t work , could you help me ?

I use the same js/ajax file for login, registration and dashboard . Can there be a conflict ?

For now I want to test only the username

phpforajax.php:

wp_localize_script(\'ajax\', \'wpAjax\', array(\'ajaxUrl\' => admin_url(\'admin-ajax.php\')));
wp_enqueue_script(\'data-login\', plugins_url(\'\', LQUSER) . \'/data/ajaxdata.js\', array(\'jquery\'), \'1.0\', true);

add_action(\'wp_ajax_add_new_user\',\'add_new_user\');
add_action(\'wp_ajax_nopriv_add_new_user\',\'add_new_user\');
function add_new_user(){
        

    if (isset($_POST[\'registr_submit\']) && wp_verify_nonce($_POST[\'csrf\'], \'csrf\')) {
        
    // User data registration
    $username       = $_POST[\'username\'];
    /*
    $firstname      = $_POST[\'firstname\'];
    $lastname       = $_POST[\'lastname\'];
    $email          = $_POST[\'email\'];
    $newpassword    = $_POST[\'newpassword\'];
    $confirmpassword= $_POST[\'confirmpassword\'];
    $roleuser       = \'subscriber\';
    verify only username
    */
if(empty($username)){
    $data = [];
    $data[\'success\'] = false;
    $data[\'message\'] = \'Username is required\';

    echo json_encode($return);
}
// before to use wp_send_json_error I must find my mistake or my forgetting
//if i use wp_die(); the form isn\'t pass to ajax
}

my_registration_form.php

    <form class="registr-form" action="" method="POST" id="my_registration_form">
                <input type="text" name="username" id="username" placeholder="Votre nom d\'utilisateur" autocomplete="username"/>
                <span id="error-emp-name"></span>
    <input type="hidden" name="csrf" value="<?php echo wp_create_nonce(\'csrf\'); ?>"/>
    <input type="submit" id="registr_submit" name="registr_submit" class="submit-registr-btn" value="Je valide">   
    </form>

ajaxdata.js:

$(function(){
            $(\'#registr_submit\').on(\'click\', function(e){
                e.preventDefault();
                $.ajax({ 
                    url     : wpAjax.ajaxUrl,
                    data    : {action:\'add_new_user\'},
                    type    : \'POST\',
                    dataType: \'json\',
                    success : function(response){
                        var data = $.parseJSON(response);
                        // below work and display input 
                        console.log(username);
                        // below doesn\'t work 
                        if(data.success == false){
                            console.log(response.message); //must display : Username is required.
                        
                  
                        }
                    }
                });
    
            });