Getting user id or other info from user_register in WP

Question

I'm trying to extract or get atleast the user_id upon registration in WP using user_register hook. But the alert doesn't even shows up.

add_action( 'user_register', 'display_id' );

function display_id( $user_id ) {
    echo '<script type="text/javascript">alert("Registration successful");</script>';
}

Answer 1

add_action( 'user_register', 'display_id', 10, 1);

function display_id( $user_id ) {
    echo '<script type="text/javascript">alert("Registration successful");</script>';
}