differentiate between input and output params inside Load (#4160)

* differentiate between input and output params inside `Load`

* add changeset

* run format

* update changelog message

Co-authored-by: Rich Harris <[email protected]>

* use `InputProps` as default value for `OutputProps`

Co-authored-by: Rich Harris <[email protected]>

* Update .changeset/yellow-geese-tap.md

Co-authored-by: Rich Harris <[email protected]>
Co-authored-by: Rich Harris <[email protected]>

Author: ivanhofer

.changeset/yellow-geese-tap.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+[breaking] allow `InputProps` and `OutputProps` to be typed separately in `Load`

packages/kit/types/index.d.ts

@@ -116,8 +116,14 @@ export interface HandleError {
  *
  * Note that you can use [generated types](/docs/types#generated-types) instead of manually specifying the Params generic argument.
  */
-export interface Load<Params = Record<string, string>, Props = Record<string, any>> {
-	(input: LoadInput<Params>): MaybePromise<Either<Fallthrough, LoadOutput<Props>>>;
+export interface Load<
+	Params extends Record<string, string> = Record<string, string>,
+	InputProps extends Record<string, any> = Record<string, any>,
+	OutputProps extends Record<string, any> = InputProps
+> {
+	(input: LoadInput<Params, InputProps>): MaybePromise<
+		Either<Fallthrough, LoadOutput<OutputProps>>
+	>;
 }
 
 export interface Navigation {

packages/kit/types/private.d.ts

@@ -220,10 +220,10 @@ export type JSONValue =
 	| JSONValue[]
 	| JSONObject;
 
-export interface LoadInput<Params = Record<string, string>> {
+export interface LoadInput<Params = Record<string, string>, Props = Record<string, any>> {
 	url: URL;
 	params: Params;
-	props: Record<string, any>;
+	props: Props;
 	fetch(info: RequestInfo, init?: RequestInit): Promise<Response>;
 	session: App.Session;
 	stuff: Partial<App.Stuff>;