feat: Unleash feature flags via src/lib/flags.tsx, hello-banner on home page

src/app.test.tsx

@@ -3,12 +3,21 @@ import { createMemoryRouter, RouterProvider } from "react-router";
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 import { routes } from "./routes";
 
+// Flags come from the Unleash SDK; tests control them directly.
+const flagsOn = vi.hoisted(() => new Set<string>());
+vi.mock("@unleash/proxy-client-react", () => ({
+  FlagProvider: ({ children }: { children: React.ReactNode }) => children,
+  useFlag: (name: string) => flagsOn.has(name),
+  useVariant: () => ({ name: "disabled", enabled: false }),
+}));
+
 function renderAt(path: string) {
   const router = createMemoryRouter(routes, { initialEntries: [path] });
   return render(<RouterProvider router={router} />);
 }
 
 beforeEach(() => {
+  flagsOn.clear();
   window.__APP_CONFIG__ = { API_BASE_URL: "/api", APP_NAME: "test-app" };
 });
 
@@ -25,6 +34,20 @@ describe("app", () => {
     expect(screen.getByTestId("build-sha")).toHaveTextContent("dev");
   });
 
+  it("home hides the banner while hello-banner is off", async () => {
+    renderAt("/");
+    await screen.findByRole("heading", { name: "test-app" });
+    expect(screen.queryByTestId("banner")).not.toBeInTheDocument();
+  });
+
+  it("home shows the banner while hello-banner is on", async () => {
+    flagsOn.add("hello-banner");
+    renderAt("/");
+    expect(await screen.findByTestId("banner")).toHaveTextContent(
+      "hello-banner flag is on",
+    );
+  });
+
   it("status fetches / and /readyz under API_BASE_URL and renders the JSON", async () => {
     const fetchMock = vi.fn(async (url: string) => {
       const body =

src/lib/config.ts

@@ -1,6 +1,10 @@
 export interface AppConfig {
   API_BASE_URL: string;
   APP_NAME: string;
+  /** Unleash base URL as seen by the browser (empty = flags off). */
+  UNLEASH_URL: string;
+  /** Unleash frontend token: public by design (read-only, one environment). */
+  UNLEASH_FRONTEND_TOKEN: string;
 }
 
 declare global {
@@ -9,7 +13,12 @@ declare global {
   }
 }
 
-const defaults: AppConfig = { API_BASE_URL: "/api", APP_NAME: "app" };
+const defaults: AppConfig = {
+  API_BASE_URL: "/api",
+  APP_NAME: "app",
+  UNLEASH_URL: "",
+  UNLEASH_FRONTEND_TOKEN: "",
+};
 
 // Runtime config comes from /config.js (window.__APP_CONFIG__), so one image
 // serves every environment. Read it at call time so tests can override it.

src/lib/flags.test.tsx

@@ -0,0 +1,93 @@
+import { render, screen } from "@testing-library/react";
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+// Mock the Unleash React SDK: capture provider props, control flag state.
+const sdk = vi.hoisted(() => ({
+  on: new Set<string>(),
+  providerProps: [] as Record<string, unknown>[],
+}));
+vi.mock("@unleash/proxy-client-react", () => ({
+  FlagProvider: (props: { children: React.ReactNode }) => {
+    sdk.providerProps.push(props);
+    return props.children;
+  },
+  useFlag: (name: string) => sdk.on.has(name),
+  useVariant: () => ({ name: "disabled", enabled: false }),
+}));
+
+import { FlagsProvider, unleashConfig, useFlag } from "./flags";
+
+function Probe() {
+  return <p>{useFlag("hello-banner") ? "on" : "off"}</p>;
+}
+
+beforeEach(() => {
+  sdk.on.clear();
+  sdk.providerProps.length = 0;
+});
+
+describe("unleashConfig", () => {
+  const base = { API_BASE_URL: "/api", APP_NAME: "web" };
+  it("is null when UNLEASH_URL or token is unset", () => {
+    expect(
+      unleashConfig({ ...base, UNLEASH_URL: "", UNLEASH_FRONTEND_TOKEN: "" }),
+    ).toBeNull();
+    expect(
+      unleashConfig({
+        ...base,
+        UNLEASH_URL: "http://u",
+        UNLEASH_FRONTEND_TOKEN: "",
+      }),
+    ).toBeNull();
+  });
+  it("points at the Frontend API", () => {
+    expect(
+      unleashConfig({
+        ...base,
+        UNLEASH_URL: "http://unleash.test/",
+        UNLEASH_FRONTEND_TOKEN: "*:development.x",
+      }),
+    ).toMatchObject({
+      url: "http://unleash.test/api/frontend",
+      clientKey: "*:development.x",
+      appName: "web",
+    });
+  });
+});
+
+describe("FlagsProvider", () => {
+  it("starts the client when configured", () => {
+    window.__APP_CONFIG__ = {
+      APP_NAME: "web",
+      UNLEASH_URL: "http://unleash.test",
+      UNLEASH_FRONTEND_TOKEN: "tok",
+    };
+    sdk.on.add("hello-banner");
+    render(
+      <FlagsProvider>
+        <Probe />
+      </FlagsProvider>,
+    );
+    expect(screen.getByText("on")).toBeInTheDocument();
+    expect(sdk.providerProps[0]).toMatchObject({
+      startClient: true,
+      config: { url: "http://unleash.test/api/frontend", clientKey: "tok" },
+    });
+  });
+
+  it("stays offline when UNLEASH_URL is unset", () => {
+    window.__APP_CONFIG__ = { APP_NAME: "web" };
+    const info = vi.spyOn(console, "info").mockImplementation(() => {});
+    render(
+      <FlagsProvider>
+        <Probe />
+      </FlagsProvider>,
+    );
+    expect(screen.getByText("off")).toBeInTheDocument();
+    expect(sdk.providerProps[0]).toMatchObject({
+      startClient: false,
+      config: { disableRefresh: true, disableMetrics: true },
+    });
+    expect(info).toHaveBeenCalledTimes(1);
+  });
+});

src/lib/flags.tsx

@@ -0,0 +1,55 @@
+import { useMemo, type ReactNode } from "react";
+import {
+  FlagProvider,
+  useFlag as useUnleashFlag,
+  useVariant as useUnleashVariant,
+  type IConfig,
+} from "@unleash/proxy-client-react";
+import { getConfig, type AppConfig } from "./config";
+
+// Offline client: no network, no metrics, every flag reads false.
+const OFFLINE: IConfig = {
+  url: "http://localhost/offline",
+  clientKey: "offline",
+  appName: "offline",
+  bootstrap: [],
+  disableRefresh: true,
+  disableMetrics: true,
+};
+
+/** Unleash Frontend API config from runtime config, or null if unset. */
+export function unleashConfig(cfg: AppConfig): IConfig | null {
+  if (!cfg.UNLEASH_URL || !cfg.UNLEASH_FRONTEND_TOKEN) return null;
+  return {
+    url: `${cfg.UNLEASH_URL.replace(/\/+$/, "")}/api/frontend`,
+    clientKey: cfg.UNLEASH_FRONTEND_TOKEN,
+    appName: cfg.APP_NAME,
+    refreshInterval: 15,
+  };
+}
+
+/**
+ * Wrap the app once (main.tsx). With UNLEASH_URL/UNLEASH_FRONTEND_TOKEN unset
+ * (local dev, tests) every flag reads false and nothing is fetched.
+ */
+export function FlagsProvider({ children }: { children: ReactNode }) {
+  const config = useMemo(() => {
+    const c = unleashConfig(getConfig());
+    if (!c) console.info("feature flags disabled: UNLEASH_URL unset");
+    return c;
+  }, []);
+  return (
+    <FlagProvider config={config ?? OFFLINE} startClient={config !== null}>
+      {children}
+    </FlagProvider>
+  );
+}
+
+/** True while the flag is on; re-renders when Unleash reports a change. */
+export function useFlag(name: string): boolean {
+  return useUnleashFlag(name);
+}
+
+export function useVariant(name: string) {
+  return useUnleashVariant(name);
+}

src/main.tsx

@@ -1,12 +1,15 @@
 import { StrictMode } from "react";
 import { createRoot } from "react-dom/client";
 import { createBrowserRouter, RouterProvider } from "react-router";
+import { FlagsProvider } from "./lib/flags";
 import { routes } from "./routes";
 
 const router = createBrowserRouter(routes);
 
 createRoot(document.getElementById("root")!).render(
   <StrictMode>
-    <RouterProvider router={router} />
+    <FlagsProvider>
+      <RouterProvider router={router} />
+    </FlagsProvider>
   </StrictMode>,
 );

src/routes/Home.tsx

@@ -1,8 +1,20 @@
 import { buildSha, getConfig } from "../lib/config";
+import { useFlag } from "../lib/flags";
 
 export function Home() {
+  // Demo flag (Plan 07): toggle "hello-banner" in Unleash, then reload.
+  const banner = useFlag("hello-banner");
   return (
     <section>
+      {banner && (
+        <p
+          role="status"
+          data-testid="banner"
+          style={{ background: "#fff3c4", padding: "0.5rem 1rem" }}
+        >
+          The hello-banner flag is on.
+        </p>
+      )}
       <h1>{getConfig().APP_NAME}</h1>
       <p>
         Build: <code data-testid="build-sha">{buildSha}</code>

src/test-setup.ts

@@ -1 +1,6 @@
 import "@testing-library/jest-dom/vitest";
+import { cleanup } from "@testing-library/react";
+import { afterEach } from "vitest";
+
+// vitest globals are off, so Testing Library can't register this itself.
+afterEach(cleanup);