i just wrote this functions:

       add_action('woocommerce_before_single_product', 'product_out_of_stock_redirect');
function product_out_of_stock_redirect(){
  global $post;
  global $product;

  $terms = get_the_terms( $post->ID, 'product_cat' );

  foreach ( $terms as $term ) {
    $product_cat_id = $term->term_id;
    break;
  }  
  $terms1 = get_the_terms( $post->ID, 'pa_marca' );

  foreach ( $terms1 as $term1 ) {
    $marca = $term1->name;
    break;
  }  
    $marca = str_replace(".","-",$marca);
    $marca = str_replace("/","-",$marca);
    $marca = str_replace("'","",$marca);
  if (!$marca) { 
    $link = get_term_link( (int)$product_cat_id, 'product_cat' );
  }
  else { $link = get_term_link( (int)$product_cat_id, 'product_cat' ); $link= $link . "?filter_marca=" . $marca;  }

    if (!$product->is_in_stock()){   
       wp_redirect( $link, 301 );
       exit(); // Always after wp_redirect() to avoid an error
    }
}

as you can see this simply functions redirects all user that visit an outofstock products to another page.
But i need to add an alert to keep user informated of that…how can i add a simply popup that alerts an user?