react-form: setting value to `undefined`

[enisdenjo]

Expected Behaviour

Setting a field value to undefined either sets the value to undefined or removes the path to the value from the values.

Actual Behaviour

Setting a field value to undefined does nothing.

Previous Reasoning

When an existing array in the form values is used through a FormField to render nested array elements which also use any derivation of the FormField component, removing items from the array itself becomes impossible. The child FormField would set the field to undefined before/instead of unmounting. Efficiently preventing any removals of the items in the array.

Say we have a form which looks like this:

const values = { people: ['John', 'Jane'] };
const tree = (
  <Form defaultValues={values}>
    <FormField path="people">
      {(array, setArray) => (
        <>
          {array.map((_0, index) => (
            <FormField path={`people[${index}]`}>
              {(item) => (
                <input manipulate={item} />
              )}
            </FormField>
          ))}
          <RemoveLast onClick={() =>  setArray(array.pop())} />
        </>
      )}
    </FormField>
  </Form>
);

After you click the <RemoveLast /> the array would become: ['John', undefined], instead of: ['John'].

Described in the following test:

domonda-js/packages/domonda-react-form/__tests__/FormField.test.tsx

Lines 330 to 383 in c18bc2f

	it('should not render or set undefined values on array fields', (done) => {
	interface DV {
	people: string[];
	}
	const dv: DV = {
	people: ['John', 'Foo', 'Bar'],
	};
	let form: DomondaForm<DV>;
	const { rerender } = render(
	<Form getForm={(f) => (form = f)} resetOnDefaultValuesChange defaultValues={dv}>
	<FormField<any[]> path="people">
	{({ value }) => (
	<>
	{value.map((person: any, index) => (
	<FormField key={person} path={`people[${index}]`}>
	{() => null}
	</FormField>
	))}
	</>
	)}
	</FormField>
	</Form>,
	);
	// @ts-ignore because form should indeed be set here
	if (!form) {
	throw new Error('form instance should be set here!');
	}
	const nextDv: DV = {
	people: ['Foo'],
	};
	rerender(
	<Form resetOnDefaultValuesChange defaultValues={nextDv}>
	<FormField<any[]> path="people">
	{({ value }) => (
	<>
	{value.map((person: any, index) => (
	<FormField key={person} path={`people[${index}]`}>
	{() => null}
	</FormField>
	))}
	</>
	)}
	</FormField>
	</Form>,
	);
	expect(form.state.values).toEqual(nextDv);
	done();
	});

and such behaviour is allowed by this segment:

domonda-js/packages/domonda-form/src/createFormField.ts

Lines 92 to 95 in c18bc2f

	values:
	rest.value === undefined
	? state.values
	: setWith(() => undefined, path, rest.value, form.state.values),