Can't pass var from php wp_ajax into ajax script : result undefined or null

Question

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.
                        
                  
                        }
                    }
                });
    
            });

Answer 1

thank you so much Tom and Sally. My issue was indeed to not send registr_submit to. In reallity I didn't well understand ajax, now it's good. I thought that serialize string of inputs form was enough to get the submit value. Mainly I convinced that php send to jquery/ajax while it is obviously the opposite. So, in order :

ajaxdata.js:

$(function(){
      $('#registr_submit').on('click', function(e){
                e.preventDefault();
                var userdata = $('#my_registration_form').serialize();
            userdata += '&action=add_new_user&regsubmit=registr_submit';
        
            var username = $('#username').val();
                $.ajax({ 
                    url     : wpAjax.ajaxUrl,
                    data    : userdata,
                    type    : 'POST',
                    dataType: 'json',
                    success : function(response){
                        if(username == ''){
                           if(response.success == false) {
                            
                              $('#error-emp-name').html(response.data);
                           }
                        }
                        else{
                            if(response.success == true) {
                                $('#error-emp-name').html('Correcte');
                            }
                        }
                        
                  
                   }
               }
          });
    
      });

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(){
        

    function add_new_user(){
    global $wpdb;       

    if (isset($_POST['regsubmit']) && wp_verify_nonce($_POST['csrf'], 'csrf')) {
        
    $username       = $_POST['username'];
    $firstname      = $_POST['firstname'];
    $lastname       = $_POST['lastname'];
    $email          = $_POST['email'];
    $newpassword    = $_POST['newpassword'];
    $confirmpassword= $_POST['confirmpassword'];
    $roleuser       = 'subscriber';
    
    if (empty($username)) {     
        wp_send_json_error('Username is required'); 
    }else{
        wp_send_json_success('Ok');
    }
    
}