fix: deep partial .value() and .set(...) types for forms (#14837)

fixes #14835 

The form's default values are an empty object `{}` when defaults are not
provided by `.set()`. Therefore, the `.value()` getter should return a
deep partial of the schema type as all values are initially undefined.

Plus, deep partial `.set()` allows setting _some_, but not _all_ values
initially.

[`DeepPartial`
Demo](https://www.typescriptlang.org/play/?noUncheckedIndexedAccess=true#code/C4TwDgpgBAIhFgAoEMBOwCWyA2AeAKgHxQC8AsAFACQ+UEAHsBAHYAmAzlAEoQDGA9qla5EqfpHQgA0hBAAaKAFdmAa2b8A7s2IAfJavVaA2gF1KVKgH4oAb3MWqRqVAzMoK2fwBmUfCcsAXL5OJnSMLBzcfILCouIQkjLy+mqa2lB6yqnGodZwCCjoWHj4IcRBpVImANz2VAC+9hUZ+qwQXq4QrLUUlALM7MBQTINB+UhomDi4dhRQ81D8AEYAVkE2wxCjsMhMUPVylAtQaKjrm9uDqK4A5vumh3MLqHxBPAJCuFe3Ct-MN4RKPViCRbJQgeCKCNgAA6ZYrSww6GUAD0KIWAD1LJRoTDToijAAGfxIrbAVHo+ZYnFkmEvXiI5EUNGY7G9Cj9QYXYAAJjG8AmRWmfwBpCgACJxRTjligA)

---

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---------

Co-authored-by: Tee Ming <[email protected]>

Author: sillvva

.changeset/nice-chairs-switch.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: use deep partial types for form remote functions `.value()` and `.set(...)`

packages/kit/src/exports/public.d.ts

@@ -16,6 +16,7 @@ import {
 	PrerenderOption,
 	RequestOptions,
 	RouteSegment,
+	DeepPartial,
 	IsAny
 } from '../types/private.js';
 import { BuildData, SSRNodeLoader, SSRRoute, ValidatedConfig } from 'types';
@@ -1910,9 +1911,9 @@ type InputElementProps<T extends keyof InputTypeMap> = T extends 'checkbox' | 'r
 
 type RemoteFormFieldMethods<T> = {
 	/** The values that will be submitted */
-	value(): T;
+	value(): DeepPartial<T>;
 	/** Set the values that will be submitted */
-	set(input: T): T;
+	set(input: DeepPartial<T>): DeepPartial<T>;
 	/** Validation issues, if any */
 	issues(): RemoteFormIssue[] | undefined;
 };

packages/kit/src/types/private.d.ts

@@ -243,4 +243,12 @@ export interface RouteSegment {
 /** @default 'never' */
 export type TrailingSlash = 'never' | 'always' | 'ignore';
 
+export type DeepPartial<T> = T extends Record<PropertyKey, unknown> | unknown[]
+	? {
+			[K in keyof T]?: T[K] extends Record<PropertyKey, unknown> | unknown[]
+				? DeepPartial<T[K]>
+				: T[K];
+		}
+	: T | undefined;
+
 export type IsAny<T> = 0 extends 1 & T ? true : false;

packages/kit/types/index.d.ts

@@ -1885,9 +1885,9 @@ declare module '@sveltejs/kit' {
 
 	type RemoteFormFieldMethods<T> = {
 		/** The values that will be submitted */
-		value(): T;
+		value(): DeepPartial<T>;
 		/** Set the values that will be submitted */
-		set(input: T): T;
+		set(input: DeepPartial<T>): DeepPartial<T>;
 		/** Validation issues, if any */
 		issues(): RemoteFormIssue[] | undefined;
 	};
@@ -2402,6 +2402,14 @@ declare module '@sveltejs/kit' {
 	/** @default 'never' */
 	type TrailingSlash = 'never' | 'always' | 'ignore';
 
+	type DeepPartial<T> = T extends Record<PropertyKey, unknown> | unknown[]
+		? {
+				[K in keyof T]?: T[K] extends Record<PropertyKey, unknown> | unknown[]
+					? DeepPartial<T[K]>
+					: T[K];
+			}
+		: T | undefined;
+  
 	type IsAny<T> = 0 extends 1 & T ? true : false;
 	interface Asset {
 		file: string;