Conditional does not work with add_filter

Question

I want the conditional to be applied and that it is only displayed on pages, but it does not work, what is wrong?

    function luc_add_cutom_fields_to_content( $content ) {

    $custom_fields = get_post_custom();

    $content .= "<div class='cta-div'>";

    if( isset( $custom_fields['luc_name'] ) ) {
         $content .= '<h3> '. $custom_fields['luc_name'][0] . '</h3>';
    }
    if( isset( $custom_fields['luc_description'] ) ) {
         $content .= '<p> ' . $custom_fields['luc_description'][0] . '</p>';
    }
    if( isset( $custom_fields['luc_action'] ) ) {
         $content .= '<a href=" ' . $custom_fields['luc_link'][0] . ' ">' . $custom_fields['luc_action'][0] . '</a>';
    }
    $content .= '</div>';

    return $content;
} 

if( is_page() ) {
add_filter( 'the_content', 'luc_add_cutom_fields_to_content' );
}

Answer 1

is_page() requires WP_Query to be initialized and I assume your code runs before this. You can use this function within the filter callback instead:

add_filter('the_content', function($c){
    return is_page() ? luc_add_cutom_fields_to_content($c) : $c;
});

Answer 2

Maybe the filter is firing well, but There is no global scope for the current post and the if(isset(...)) can't be execute.

At the top of the function, you can do it with get_post_meta()

 global $post;

 $custom_fields = get_post_meta($post->ID);