chore: Compatible with createRouter where basename does not start with / (#11671)

Author: nanianlisao

.changeset/tender-sheep-bake.md

@@ -0,0 +1,5 @@
+---
+"react-router": patch
+---
+
+Fix `basename` usage without a leading slash in data routers

contributors.yml

@@ -274,6 +274,7 @@
 - mtendekuyokwa19
 - mtliendo
 - namoscato
+- nanianlisao
 - ned-park
 - nenene3
 - ngbrown

packages/react-router/__tests__/router/router-test.ts

@@ -237,6 +237,38 @@ describe("a router", () => {
       });
     });
 
+    it("supports a basename prop for route matching without a leading slash", async () => {
+      let history = createMemoryHistory({
+        initialEntries: ["/base/name/path"],
+      });
+      let router = createRouter({
+        basename: "base/name",
+        routes: [{ path: "path" }],
+        history,
+      });
+      expect(router.state).toMatchObject({
+        location: {
+          hash: "",
+          key: expect.any(String),
+          pathname: "/base/name/path",
+          search: "",
+          state: null,
+        },
+        matches: [
+          {
+            params: {},
+            pathname: "/path",
+            pathnameBase: "/path",
+            route: {
+              id: "0",
+              path: "path",
+            },
+          },
+        ],
+        initialized: true,
+      });
+    });
+
     it("supports subscribers", async () => {
       let history = createMemoryHistory({ initialEntries: ["/"] });
       let count = 0;

packages/react-router/lib/router/router.ts

@@ -867,6 +867,9 @@ export function createRouter(init: RouterInit): Router {
   );
   let inFlightDataRoutes: AgnosticDataRouteObject[] | undefined;
   let basename = init.basename || "/";
+  if (!basename.startsWith("/")) {
+    basename = `/${basename}`;
+  }
   let dataStrategyImpl = init.dataStrategy || defaultDataStrategyWithMiddleware;
 
   // Config driven behavior flags