fix: getFallbacks use fallback value of an object

[muningis]

Resolves #1150

This is, semi breaking change though I would assume, as it changes default behaviour.
Previously, it would check for fallbacks of an object, and ignore if it has a fallback itself, eg.:

const ObjectWithFallback = v.fallback(
  v.object({ foo: string() }), <-- checks if entries of this object has a fallback
  { foo: "bar" }, <-- completely ignored
);

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Name	Status	Preview	Comments	Updated (UTC)
valibot	❌ Failed (Inspect)			Apr 13, 2025 7:20am

[muningis]

Ideally, it would be like this, however fallback requires all values to be defined.

Suggested change

	// If it's and object schema and it has a fallback, return it
	if (schema.type === 'object' && 'fallback' in schema) {
	return schema.fallback as InferFallbacks<TSchema>;
	}
	// If it is an object schema, return fallbacks of entries
	if ('entries' in schema) {
	const object: Record<string, unknown> = {};
	// If it is an object schema, return fallbacks of entries
	// If an object itself has a fallback, use it
	if ('entries' in schema) {
	const object: Record<string, unknown> = schema.type === 'object' && 'fallback' in schema ? schema.fallback : {};

eg.

const ObjectWithFallbackSchema = fallback(object({
  foo: number(),
  bar: fallback(number(), 5)
}), {
  foo: 3
});

I would expect bar to not be required, because it itself has a fallback.

[muningis]

It can be achieve by adjusting Fallback type to

/**
 * Fallback type.
 */
export type Fallback<
  TSchema extends BaseSchema<unknown, unknown, BaseIssue<unknown>>,
  TFallbackSchema = MaybeReadonly<InferOutput<TSchema>>
> =
  | {
    [key in keyof TFallbackSchema as TFallbackSchema[key] extends SchemaWithFallback<any, any> ? never : key]: TFallbackSchema[key];
    } & {
      [key in keyof TFallbackSchema as TFallbackSchema[key] extends SchemaWithFallback<any, any> ? key : never]?: TFallbackSchema[key];
    }
  | ((
      dataset?: OutputDataset<InferOutput<TSchema>, InferIssue<TSchema>>,
      config?: Config<InferIssue<TSchema>>
    ) => ({
      [key in keyof TFallbackSchema]: TFallbackSchema[key] extends SchemaWithFallback<any, any>
      ? TFallbackSchema[key]
      : TFallbackSchema[key];
    } & {
      [key in keyof TFallbackSchema as TFallbackSchema[key] extends SchemaWithFallback<any, any> ? key : never]?: TFallbackSchema[key];
    }));

but that's whole other discussion

[fabian-hiller]

Isn't it enough to just check for 'fallback' in schema? This way we make it the default behaviour for any schema.

[muningis]

Yeah, could be done that way too.

But probably this if clause can be removed, and do this when retrieving fallbacks of an object -> #1155 (comment)

[fabian-hiller]

I think that we also need to update the types in library/src/methods/getFallbacks/types.ts to used the exact fallback if available.

[muningis]

Good point, definitely need to update the type too. Will do, once decided if fix/change should be implemented/added or not.

[fabian-hiller]

If we merge this. Do we consider this a fix or a breaking change and park it in a new v2 branch? I am not really sure. I really wonder why I implemented it the way I did.

[muningis]

IMO it’s breaking change even if it’s a bugfix. Libraries can’t (shouldn’t be able to) afford such changes with minor/patch releases.