Function watch

src/logic/createFormControl.ts:1288–1328

(
    name?:
      | FieldPath<TFieldValues>
      | ReadonlyArray<FieldPath<TFieldValues>>
      | WatchObserver<TFieldValues>,
    defaultValue?: DeepPartial<TFieldValues>,
  )

  };

  const watch: UseFormWatch<TFieldValues> = (
    name?:
      | FieldPath<TFieldValues>
      | ReadonlyArray<FieldPath<TFieldValues>>
      | WatchObserver<TFieldValues>,
    defaultValue?: DeepPartial<TFieldValues>,
  ) => {
    if (isFunction(name)) {
      _valuesSubscriberCount++;
      const { unsubscribe } = _subjects.state.subscribe({
        next: (payload) =>
          'values' in payload &&
          name(
            payload.values || _getWatch(undefined, defaultValue),
            payload as {
              name?: FieldPath<TFieldValues>;
              type?: EventType;
              value?: unknown;
            },
          ),
      });
      let called = false;

      return {
        unsubscribe: () => {
          if (called) {
            return;
          }

          called = true;
          _valuesSubscriberCount--;
          unsubscribe();
        },
      };
    }
    return _getWatch(
      name as InternalFieldName | InternalFieldName[],
      defaultValue,
      true,
    );
  };

  const _subscribe: FromSubscribe<TFieldValues> = (props) => {
    const needsValues = !!(props.formState as Record<string, unknown>)?.values;