z.discriminatedUnion and z.string.array throw SchemaError when submitting

[endygower]

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Description
while I read that discriminatedUnion might be deprecated (here). however, this issue also reproducible using valibot.variant. I don't know whether valibot might be deprecated as well

I just so happen to write this using zod

Using z.discriminatedUnion and z.string().array() ie:

const schema = z.discriminatedUnion('type', [
	z.object({ type: z.literal('empty') }),
	z.object({ type: z.literal('extra'), options: z.string().array() })
])

it throws SchemaError on the server

However, it works when using z.string()

const schema1 = z.discriminatedUnion('type', [
	z.object({ type: z.literal('empty') }),
	z.object({ type: z.literal('extra'), options: z.string() })
])

Investigation
I did some investigation and found the responsible code is in this line

sveltekit-superforms/src/lib/formData.ts

Lines 201 to 209 in f2201d1

	const defaultPropertyType =
	typeof schema.additionalProperties == 'object'
	? schema.additionalProperties
	: ({ type: 'string' } as const);
	for (const key of schemaKeys) {
	const property: JSONSchema7Definition = schema.properties
	? schema.properties[key]
	: defaultPropertyType;

when parsing the form data, property is set as defaultPropertyType which is a { type: 'string' }

it doesn't consider that on top level union the schema is as follow and doesn't have properties

{
  $schema: ...,
  anyOf: [...]
}

then it will call parseSingleValue and the output will be

const data = new FormData()
data.append('options', '1')
data.append('options', '2')

imaginaryCallTo_parseFormData(data, ...)
// actual: { options: '2' }
// expected: { options: ['1', '2'] }

which will always fail the zod schema and later throws SchemaError

If applicable, a MRE
inside +page.server.ts you can change the schema use between schema (z.string.array) and schema1 (z.string)
https://www.sveltelab.dev/4adylfnc6g7pv98

[ciscoheat]

Thank you for a very thorough investigation, will hopefully fix this for the next release.

[ciscoheat]

Can you just confirm that the same thing happens when dataType is set to 'json' (or not) for the form?

[endygower]

I just checked it sends the form correctly using dataType json.
I also updated the MRE if you want to try it yourself

Meanwhile, I will use dataType json while it is being fixed. thanks @ciscoheat

[ciscoheat]

Ok, then the "fix" will be to issue an error, since unions aren't allowed unless dataType is set to json.