fix(router): route("/", sub) mounts at the root, not double-slash paths

.changeset/router-root-mount.md

@@ -0,0 +1,5 @@
+---
+"@chttp/router": patch
+---
+
+fix(router): `route("/", sub)` now mounts the sub-app at the root instead of registering double-slash paths (`//health`) that 404 on every request. A `"/"` prefix collapses to `""` before `mountRoutes` concatenation, matching Hono behavior.

packages/router/src/index.ts

@@ -1698,7 +1698,13 @@ export class App {
 
   route(prefix: string, sub: App): App {
     let p = String(prefix);
-    if (p.length > 1 && p.charAt(p.length - 1) === "/") {
+    // A "/" prefix mounts at the root (issue #97): mountRoutes concatenates
+    // prefix + sub-path verbatim, so "/" must collapse to "" instead of
+    // registering "//health"-style double-slash paths. Its empty-path guard
+    // then turns "" + "" into "/" for root handlers.
+    if (p === "/") {
+      p = "";
+    } else if (p.length > 1 && p.charAt(p.length - 1) === "/") {
       p = String(p.slice(0, p.length - 1));
     }
     mountRoutes(sub, this, p, copyMw(this._mw));

packages/router/test/router.test.js

@@ -313,6 +313,44 @@ test("route() mounts a sub-app under a prefix preserving params", () => {
   assert.equal(call(app, { url: "/api/" }).body, "subroot");
 });
 
+test('route("/", sub) mounts sub-app routes at the root (issue #97)', () => {
+  const sub = createRouter();
+  sub.get("/health", (c) => c.text("ok"));
+  const app = createRouter();
+  app.route("/", sub);
+  const res = call(app, { url: "/health" });
+  assert.equal(res.statusCode, 200);
+  assert.equal(res.body, "ok");
+});
+
+test('route("/", sub) resolves a handler registered at "/" in the sub-app', () => {
+  const sub = createRouter();
+  sub.get("/", (c) => c.text("subroot"));
+  const app = createRouter();
+  app.route("/", sub);
+  const res = call(app, { url: "/" });
+  assert.equal(res.statusCode, 200);
+  assert.equal(res.body, "subroot");
+});
+
+test('route("/", sub) preserves params through the root mount', () => {
+  const sub = createRouter();
+  sub.get("/item/:id", (c) => c.json({ id: c.req.param("id") }));
+  const app = createRouter();
+  app.route("/", sub);
+  assert.equal(JSON.parse(call(app, { url: "/item/7" }).body).id, "7");
+  assert.equal(call(app, { url: "/missing" }).statusCode, 404);
+});
+
+test('route("/api", sub) still mounts under the prefix after the root fix', () => {
+  const sub = createRouter();
+  sub.get("/users", (c) => c.text("users"));
+  const app = createRouter();
+  app.route("/api", sub);
+  assert.equal(call(app, { url: "/api/users" }).body, "users");
+  assert.equal(call(app, { url: "/users" }).statusCode, 404);
+});
+
 test("route() inherits middleware from both sides", () => {
   const hits = [];
   const sub = createRouter();