Javascript Textarea prevent Alt gr + key combinations on KeyDown/KeyUp events itself

In my text area im using onKeydown event to listen to characters typed by user.
I need to prevent Alt gr + key combination values (ē, r̥, ṭ,…) to avoid accented keys in my text area.

if (event.code === 'AltRight')  {
      console.log('Prevent Accent letters');
      event.preventDefault();
    }

(or)

if (event.code === 17 || event.code === 18)  {
      console.log('Prevent Accent letters');
      event.preventDefault();
    }

Nothing worked. They are entering these if condition but still the accent text is getting printed in my textarea. How can this be prevented