Why should setting a field to undefined result in it taking the default value?

[jashworth]

continued from this issue

That's the intended behaviour, when a field is undefined it takes the default value

(with reference to this codesandbox)

If I understand correctly, then essentially

field.handleChange(undefined); // line 171
// and the alternatives...
field.setValue(undefined);
form.resetField("schedule", {
  type: "once",
  // runDate omitted
});

all mean "reset the runDate to the default value", not to undefined, even though undefined (or omitted) has a legitimate meaning.

What's the rationale behind that decision? (instead of being explicit with reset in the API, for example)

Is there a recommended approach one should consider in this situation--where the aim is to set the value to undefined rather than to the default value?

I suppose one approach would be to set the defaultValues to a value whose "clearable" props—the ones you want to be able to omit or to set to undefined—are cleared initially:

  const initialDefaultValues = useMemo(() => {
    const defaults = structuredClone(task);
    if (defaults.schedule?.type === "once") {
      const { runDate, ...other } = defaults.schedule;
      defaults.schedule = other;
    }
    return defaults;
  }, [task]);

  const form = useForm({
    defaultValues: initialDefaultValues,
    onSubmit: ({ value }) => {
      console.log("submitting", value);
    },
  });

and then to set the actual value you want to show in a useEffect

  // 🤢🤢
  useEffect(() => {
    if (task.schedule.type === "once" && task.schedule.runDate) {
      form.setFieldValue("schedule.runDate", task.schedule.runDate, {
        dontUpdateMeta: true, // treat it as untouched
      });
    }
  }, [form, task]);

but this seems obscure, at best.

edited: for clarity