feat: support overwriting default 404 handler (#3081)

Fixes #1918

Author: marvinhagemeister

src/app.ts

@@ -242,7 +242,13 @@ export class App<State> {
     return this;
   }
 
-  handler(): (
+  /**
+   * Create handler function for `Deno.serve` or to be used in
+   * testing.
+   * @param {() => Promise<Response>} [nextFn] overwrite default 404 handler
+   * @returns {Deno.ServeHandler}
+   */
+  handler(nextFn?: () => Promise<Response>): (
     request: Request,
     info?: Deno.ServeHandlerInfo,
   ) => Promise<Response> {
@@ -273,7 +279,7 @@ export class App<State> {
 
       const next = matched.patternMatch && !matched.methodMatch
         ? DEFAULT_NOT_ALLOWED_METHOD
-        : DEFAULT_NOT_FOUND;
+        : nextFn ?? DEFAULT_NOT_FOUND;
 
       const { params, handlers, pattern } = matched;
       const ctx = new FreshReqContext<State>(

src/app_test.tsx

@@ -532,3 +532,15 @@ Deno.test("FreshApp - adding Island should convert to valid export names", () =>
     fn: component3,
   });
 });
+
+Deno.test("FreshApp - overwrite default 404 handler", async () => {
+  const app = new App().get("/foo", () => new Response("foo"));
+
+  const server = new FakeServer(
+    app.handler(() => Promise.resolve(new Response("bar"))),
+  );
+
+  const res = await server.get("/invalid");
+  const text = await res.text();
+  expect(text).toEqual("bar");
+});