code: function cvv(event) {
      var element = event.element
        , value = element.value
        , key = this.digit(event);
  
      //
      // Input must be numerical.
      //
      if (!key.allowed && event.type !== 'blur') return;
  
      //
      // Check if the number is valid.
      //
      if (this.validate && (value.length >= 3 || key.removal)) {
        this.cvv.set(
          'className',
          this.lib.parse(this.number.get('value')).cvv === value.length ? 'valid' : 'invalid'
        );
      }
    }
docs: On cvv changes check the content and validate.
  
  @param {Event} event
  @api private