feat: Unleash feature flags via src/lib/flags.ts, hello-banner on GET /

Ported from service-template-express 9b63993.

src/lib/flags.test.ts

@@ -0,0 +1,86 @@
+import { EventEmitter } from "node:events";
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import pino from "pino";
+
+// Mock the Unleash SDK: no network, full control over flag state.
+const sdk = vi.hoisted(() => ({
+  initialize: vi.fn(),
+  enabled: new Set<string>(),
+}));
+vi.mock("unleash-client", () => ({ initialize: sdk.initialize }));
+
+import { createFlags } from "./flags.js";
+
+function fakeUnleash() {
+  const emitter = new EventEmitter();
+  return Object.assign(emitter, {
+    isEnabled: vi.fn((name: string) => sdk.enabled.has(name)),
+    getVariant: vi.fn((name: string, _ctx: unknown, fallback: unknown) =>
+      sdk.enabled.has(name)
+        ? { name: "blue", enabled: true, feature_enabled: true }
+        : fallback,
+    ),
+    destroy: vi.fn(),
+  });
+}
+
+const base = {
+  SERVICE_NAME: "test-svc",
+  UNLEASH_REFRESH_MS: 5000,
+} as const;
+
+describe("createFlags", () => {
+  beforeEach(() => {
+    sdk.initialize.mockReset();
+    sdk.enabled.clear();
+  });
+
+  it("without UNLEASH_URL: all flags off, logs once, never touches the SDK", () => {
+    const lines: { msg: string }[] = [];
+    const logger = pino(
+      {},
+      { write: (l: string) => lines.push(JSON.parse(l)) },
+    );
+    const flags = createFlags(base, logger);
+    expect(flags.isEnabled("hello-banner")).toBe(false);
+    expect(flags.isEnabled("hello-banner")).toBe(false);
+    expect(flags.getVariant("hello-banner").enabled).toBe(false);
+    expect(sdk.initialize).not.toHaveBeenCalled();
+    expect(lines.filter((l) => /flags disabled/.test(l.msg))).toHaveLength(1);
+  });
+
+  it("with UNLEASH_URL: initializes the SDK and delegates", async () => {
+    const unleash = fakeUnleash();
+    sdk.initialize.mockReturnValue(unleash);
+    const flags = createFlags(
+      {
+        ...base,
+        UNLEASH_URL: "http://unleash.test/api",
+        UNLEASH_API_TOKEN: "*:development.abc",
+      },
+      pino({ level: "silent" }),
+    );
+    expect(sdk.initialize).toHaveBeenCalledWith({
+      url: "http://unleash.test/api",
+      appName: "test-svc",
+      customHeaders: { Authorization: "*:development.abc" },
+      refreshInterval: 5000,
+    });
+
+    expect(flags.isEnabled("hello-banner")).toBe(false);
+    sdk.enabled.add("hello-banner");
+    expect(flags.isEnabled("hello-banner", { userId: "u1" })).toBe(true);
+    expect(unleash.isEnabled).toHaveBeenLastCalledWith(
+      "hello-banner",
+      { userId: "u1" },
+      false,
+    );
+    expect(flags.getVariant("hello-banner").name).toBe("blue");
+
+    // SDK errors are logged, not thrown (no unhandled 'error' event).
+    expect(() => unleash.emit("error", new Error("down"))).not.toThrow();
+
+    await flags.close();
+    expect(unleash.destroy).toHaveBeenCalled();
+  });
+});