Applying a Custom Tooltip to a Line Graph Widget

I am trying to apply a custom tooltip to a chart following the steps here https://www.chartjs.org/docs/latest/samples/tooltip/content.html My options configuration protected static ?array $options = [ 'plugins' => [ 'tooltip' => [ 'callbacks' => [ 'footer' => 'customFooter' ] ] ] ]; My custom footer function which is included in the blade file that the line graph is included as a livewire component
<script> const customFooter = (tooltipItems) => { let footerString = ''; tooltipItems.forEach(function(tooltipItem) { footerString = footerString.concat(": ", tooltipItem.parsed.y); }); return 'Footer: ' + footerString; }; </script> However I get the error in the attached file, the s.footer.apply is not a function

Custom Tooltip Content | Chart.js

Open source HTML5 Charts for your website