How to delete a cookie?

Try this:

function delete_cookie( name, path, domain ) {
  if( get_cookie( name ) ) {
    document.cookie = name + "=" +
      ((path) ? ";path="+path:"")+
      ((domain)?";domain="+domain:"") +
      ";expires=Thu, 01 Jan 1970 00:00:01 GMT";
  }
}

You can define get_cookie() like this:

function get_cookie(name){
    return document.cookie.split(';').some(c => {
        return c.trim().startsWith(name + '=');
    });
}