// ...
hostedFields: {
  onFieldEvent: function (event) {
    if (event.type === 'focus') {
      // Handle focus
    } else if (event.type === 'blur') {
      // Handle blur
    } else if (event.type === 'fieldStateChange') {
      // Handle a change in validation or card type
      console.log(event.isValid); // true|false
      if (event.card) {
        console.log(event.card.type);
        // visa|master-card|american-express|diners-club|discover|jcb|unionpay|maestro
      }
    }
  }
}
// ...