Here’s a complete walkthrough for creating an Ajax-based login system in WordPress, ensuring users can log in without page reloads:

1. Modify functions.php

Head to Appearance > Theme Editor and open functions.php. Add the following code snippet:

function enqueue_custom_scripts() {
    wp_enqueue_script('jquery');
    wp_enqueue_script('custom-scripts', get_stylesheet_directory_uri() . '/js/custom-scripts.js', array('jquery'), '1.0', true);
    wp_localize_script('custom-scripts', 'ajax_login_object', array(
        'ajax_url' => admin_url('admin-ajax.php'),
        'redirect_url' => home_url()
    ));
}
add_action('wp_enqueue_scripts', 'enqueue_custom_scripts');

function ajax_login() {
    check_ajax_referer('ajax-login-nonce', 'security');

    $info = array(
        'user_login'    => $_POST['username'],
        'user_password' => $_POST['password'],
        'remember'      => true
    );

    $user_signin = wp_signon($info, false);

    if (is_wp_error($user_signin)) {
        echo json_encode(array('loggedin' => false, 'message' => __('Invalid username or password.', 'text-domain')));
    } else {
        echo json_encode(array('loggedin' => true, 'message' => __('Welcome back!', 'text-domain')));
    }

    die();
}
add_action('wp_ajax_ajax_login', 'ajax_login');
add_action('wp_ajax_nopriv_ajax_login', 'ajax_login');

2. Create JavaScript File

Inside your theme's /js folder, create custom-scripts.js and add the following code:

jQuery(document).ready(function($) {
    $(document).on('submit', '#login-form', function(e) {
        e.preventDefault();

        var $form = $(this),
            $submit = $form.find('input[type="submit"]'),
            $msg = $form.find('.login-message');

        $submit.attr('disabled', true);

        $.ajax({
            type: 'POST',
            url: ajax_login_object.ajax_url,
            data: {
                action: 'ajax_login',
                username: $form.find('#username').val(),
                password: $form.find('#password').val(),
                security: $('#ajax-login-nonce').val()
            },
            success: function(res) {
                var data = JSON.parse(res);
                $msg.html('<p class="' + (data.loggedin ? 'login-success' : 'login-error') + '">' + data.message + '</p>');

                if (data.loggedin) {
                    window.location.href = ajax_login_object.redirect_url;
                }
                $submit.removeAttr('disabled');
            },
            error: function() {
                $msg.html('<p class="login-error">Login failed. Please try again.</p>');
                $submit.removeAttr('disabled');
            }
        });
    });
});

3. HTML Login Form

Use this form where you want the login box to appear:

<form id="login-form" method="post">
    <input type="text" id="username" name="username" placeholder="Username" required>
    <input type="password" id="password" name="password" placeholder="Password" required>
    <?php wp_nonce_field('ajax-login-nonce', 'ajax-login-nonce'); ?>
    <input type="submit" value="Log In">
    <div class="login-message"></div>
</form>

Pro Tip: Always test login functionality with both valid and invalid credentials, and use a child theme to ensure your modifications aren’t lost on update.