event.detail.contactformid returning undefined

Here’s my code. event.detail.contactformid is returning as undefined. I do not want to use this code just wrote it for debugging. I got the guidance for event.detail.contactformid from: https://contactform7.com/dom-events/

add_action( 'wp_footer', 'mycustom_wp_footer' );

function mycustom_wp_footer() {
?>
<script type="text/javascript">
document.getElementById("exp-check").addEventListener( 'click', function( event ) {
var some = event.detail.contactFormId
alert(some) }
    , false);
</script>
<?php
}