h3js · pi0 · Jun 27, 2025 · Jun 27, 2025 · Jun 27, 2025 · Jun 27, 2025
@@ -1,9 +1,9 @@
-import { H3, serve, redirect, withBase } from "h3";
+import { H3, serve, redirect } from "h3";
 
 const nestedApp = new H3().get("/test", () => "/test (sub app)");
 
 const app = new H3()
   .get("/", (event) => redirect(event, "/api/test"))
-  .all("/api/**", withBase("/api", nestedApp.handler));
+  .mount("/api", nestedApp);
 
 serve(app);
@@ -104,13 +104,32 @@
       });
     }
 
-    mount(base: string, input: FetchHandler | { fetch: FetchHandler }): H3Type {
-      const fetchHandler = "fetch" in input ? input.fetch : input;
-      this.all(`${base}/**`, (event) => {
-        const url = new URL(event.url);
-        url.pathname = url.pathname.slice(base.length) || "/";
-        return fetchHandler(new Request(url, event.req));
-      });
+    mount(
+      base: string,
+      input: FetchHandler | { fetch: FetchHandler } | H3Type,
+    ): H3Type {
+      if ("handler" in input) {
+        if (input._middleware.length > 0) {
+          this._middleware.push((event, next) => {
+            return event.url.pathname.startsWith(base)
+              ? callMiddleware(event, input._middleware, next)
+              : next();
+          });
+        }
+        for (const r of input._routes) {
+          this._addRoute({
+            ...r,
+            route: base + r.route,
+          });
+        }
+      } else {
+        const fetchHandler = "fetch" in input ? input.fetch : input;
+        this.all(`${base}/**`, (event) => {
+          const url = new URL(event.url);
+          url.pathname = url.pathname.slice(base.length) || "/";
+          return fetchHandler(new Request(url, event.req));
+        });
+      }
       return this as unknown as H3Type;
     }
 

@@ -69,7 +69,7 @@ export declare class H3 {
   /**
    * @internal
    */
-  _routes?: H3Route[];
+  _routes: H3Route[];
 
   /**
    * H3 instance config.
@@ -121,9 +121,13 @@ export declare class H3 {
   handler(event: H3Event): unknown | Promise<unknown>;
 
   /**
-   * Mount a `.fetch` compatible server (like Hono or Elysia) to the H3 app.
+   * Mount an H3 app or a `.fetch` compatible server (like Hono or Elysia) with a base prefix.
+   *
+   * When mounting a sub-app, all routes will be added with base prefix and global middleware will be added as one prefixed middleware.
+   *
+   * **Note:** Sub-app options and global hooks are not inherited by the mounted app please consider setting them in the main app directly.
    */
-  mount(base: string, input: FetchHandler | { fetch: FetchHandler }): this;
+  mount(base: string, input: FetchHandler | { fetch: FetchHandler } | H3): this;
 
   /**
    * Register a global middleware.

@@ -0,0 +1,57 @@
+import { H3 } from "../src/h3.ts";
+import { describeMatrix } from "./_setup.ts";
+
+describeMatrix("mount", (t, { it, expect, describe }) => {
+  describe("mount fetch", () => {
+    it("works with fetch function passed", async () => {
+      t.app.mount("/test", (req) => new Response(new URL(req.url).pathname));
+      expect(await t.fetch("/test").then((r) => r.text())).toBe("/");
+      expect(await t.fetch("/test/").then((r) => r.text())).toBe("/");
+      expect(await t.fetch("/test/123").then((r) => r.text())).toBe("/123");
+    });
+
+    it("works with compat object", async () => {
+      t.app.mount("/test", {
+        fetch: (req: Request) => new Response(new URL(req.url).pathname),
+      });
+      expect(await t.fetch("/test/123").then((r) => r.text())).toBe("/123");
+    });
+  });
+
+  describe("mount H3", () => {
+    it("works with H3 handler", async () => {
+      t.app.mount(
+        "/test",
+        new H3()
+          .use((event) => {
+            event.res.headers.set("x-test", "1");
+          })
+          .use((event) => {
+            if (event.url.pathname === "/test/intercept") {
+              return "intercepted";
+            }
+          })
+          .get("/**:slug", (event) => ({
+            url: event.url.pathname,
+            slug: event.context.params?.slug,
+          })),
+      );
+
+      expect(t.app._routes).toHaveLength(1);
+      expect(t.app._routes[0].route).toBe("/test/**:slug");
+
+      expect(t.app._middleware).toHaveLength(1);
+
+      const res = await t.fetch("/test/123");
+      expect(res.headers.get("x-test")).toBe("1");
+      expect(await res.json()).toMatchObject({
+        url: "/test/123",
+        slug: "123",
+      });
+
+      const interceptRes = await t.fetch("/test/intercept");
+      expect(interceptRes.headers.get("x-test")).toBe("1");
+      expect(await interceptRes.text()).toBe("intercepted");
+    });
+  });
+});