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do not user javascript body class method sometimes it will give issue due to cache
<?php
function detect_login() {
$isLoggedIn = is_user_logged_in();
wp_register_script( 'detect_login', '' );
wp_enqueue_script( 'detect_login');
wp_add_inline_script( 'detect_login', "var isLoggedIn = $isLoggedIn" );
}
add_action( 'wp_enqueue_scripts', 'detect_login' );
to get the value :
window.isLoggedIn