allow to type Params of RequestHandler (#3989)

* allow to type `Params` of `RequestHandler`

* mark change as breaking

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

Author: ivanhofer

Diff for: .changeset/proud-cooks-relax.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+[breaking] allow providing `Params` type argument for `RequestHandler`

Diff for: packages/kit/types/index.d.ts

@@ -340,10 +340,10 @@ export interface PrerenderErrorHandler {
 	}): void;
 }
 
-export interface RequestEvent {
+export interface RequestEvent<Params = Record<string, string>> {
 	request: Request;
 	url: URL;
-	params: Record<string, string>;
+	params: Params;
 	locals: App.Locals;
 	platform: Readonly<App.Platform>;
 }
@@ -354,8 +354,8 @@ export interface RequestEvent {
  * that method. Note that since 'delete' is a reserved word in
  * JavaScript, delete handles are called 'del' instead.
  */
-export interface RequestHandler<Output extends Body = Body> {
-	(event: RequestEvent): MaybePromise<
+export interface RequestHandler<Params = Record<string, string>, Output extends Body = Body> {
+	(event: RequestEvent<Params>): MaybePromise<
 		Either<Output extends Response ? Response : EndpointOutput<Output>, Fallthrough>
 	>;
 }