test(islands): snapshot public API exports and signatures

frontend/src/core/islands/__tests__/main.test.ts

@@ -0,0 +1,176 @@
+/* Copyright 2026 Marimo. All rights reserved. */
+
+import { beforeEach, describe, expect, expectTypeOf, it, vi } from "vitest";
+import type * as MainModule from "../main";
+
+// `main.ts` is the public islands entry point (see islands/vite.config.mts),
+// so this suite snapshots its export surface + signatures and covers each
+// entry point. The real bridge/bootstrap spin up a Web Worker and the plugin
+// runtime, so we mock them.
+const mockInitializeApps = vi.fn(() => Promise.resolve());
+const mockStopSession = vi.fn((_appId?: string) => Promise.resolve());
+const mockInitializeIslands = vi.fn(() => Promise.resolve());
+const mockParseMarimoIslandApps = vi.fn(() => [] as Array<{ id: string }>);
+
+vi.mock("@/core/islands/bridge", () => ({
+  getGlobalBridge: () => ({
+    initializeApps: mockInitializeApps,
+    stopSession: mockStopSession,
+  }),
+}));
+
+vi.mock("@/core/islands/bootstrap", () => ({
+  initializeIslands: mockInitializeIslands,
+}));
+
+vi.mock("@/core/islands/parse", () => ({
+  parseMarimoIslandApps: mockParseMarimoIslandApps,
+}));
+
+/**
+ * Re-import a fresh copy of `main.ts` so the module-level `bootstrapPromise`
+ * memoization does not leak between tests. The module auto-runs `initialize()`
+ * on load, so we clear the DOM first to keep that a no-op.
+ */
+async function importMain(): Promise<typeof MainModule> {
+  vi.resetModules();
+  return import("../main");
+}
+
+beforeEach(() => {
+  document.body.replaceChildren();
+  mockInitializeApps.mockClear();
+  mockStopSession.mockClear();
+  mockInitializeIslands.mockClear();
+  mockParseMarimoIslandApps.mockReset();
+  mockParseMarimoIslandApps.mockReturnValue([]);
+});
+
+describe("islands public API surface", () => {
+  it("exposes a stable set of exports and signatures", async () => {
+    const mod = await importMain();
+    const surface = Object.fromEntries(
+      Object.keys(mod)
+        .toSorted()
+        .map((key) => {
+          const value = (mod as Record<string, unknown>)[key];
+          return [
+            key,
+            typeof value === "function"
+              ? `function(arity ${value.length})`
+              : typeof value,
+          ];
+        }),
+    );
+
+    expect(surface).toMatchInlineSnapshot(`
+      {
+        "canReplaceApp": "function(arity 0)",
+        "initialize": "function(arity 0)",
+        "stopApp": "function(arity 1)",
+      }
+    `);
+  });
+
+  it("matches the declared TypeScript signatures", () => {
+    expectTypeOf<typeof MainModule.canReplaceApp>().toEqualTypeOf<
+      () => boolean
+    >();
+    expectTypeOf<typeof MainModule.initialize>().toEqualTypeOf<
+      () => Promise<void>
+    >();
+    expectTypeOf<typeof MainModule.stopApp>().toEqualTypeOf<
+      (appId?: string) => Promise<void>
+    >();
+  });
+});
+
+describe("canReplaceApp", () => {
+  it("returns false when the document has no island element", async () => {
+    const mod = await importMain();
+
+    expect(mod.canReplaceApp()).toBe(false);
+    // Short-circuits on the DOM guard before parsing.
+    expect(mockParseMarimoIslandApps).not.toHaveBeenCalled();
+  });
+
+  it("returns true for exactly one app without materializing the DOM", async () => {
+    const mod = await importMain();
+    document.body.innerHTML = `<marimo-island data-app-id="app-1"></marimo-island>`;
+    mockParseMarimoIslandApps.mockReturnValue([{ id: "app-1" }]);
+
+    expect(mod.canReplaceApp()).toBe(true);
+    // A probe must not mutate island attributes.
+    expect(mockParseMarimoIslandApps).toHaveBeenCalledWith(document, {
+      materialize: false,
+    });
+  });
+
+  it("returns false when more than one app is present", async () => {
+    const mod = await importMain();
+    document.body.innerHTML = `<marimo-island data-app-id="app-1"></marimo-island>`;
+    mockParseMarimoIslandApps.mockReturnValue([
+      { id: "app-1" },
+      { id: "app-2" },
+    ]);
+
+    expect(mod.canReplaceApp()).toBe(false);
+  });
+
+  it("returns false when the island parses into zero apps", async () => {
+    const mod = await importMain();
+    document.body.innerHTML = `<marimo-island></marimo-island>`;
+    mockParseMarimoIslandApps.mockReturnValue([]);
+
+    expect(mod.canReplaceApp()).toBe(false);
+  });
+});
+
+describe("stopApp", () => {
+  it("forwards the requested app id to the bridge", async () => {
+    const mod = await importMain();
+
+    await mod.stopApp("app-7");
+    expect(mockStopSession).toHaveBeenCalledWith("app-7");
+
+    await mod.stopApp();
+    expect(mockStopSession).toHaveBeenLastCalledWith(undefined);
+  });
+});
+
+describe("initialize", () => {
+  it("skips bootstrap and app start when no islands are present", async () => {
+    const mod = await importMain();
+
+    await mod.initialize();
+
+    expect(mockInitializeIslands).not.toHaveBeenCalled();
+    expect(mockInitializeApps).not.toHaveBeenCalled();
+  });
+
+  it("bootstraps once, marks islands, and starts apps on each call", async () => {
+    const mod = await importMain();
+    document.body.innerHTML = `<marimo-island></marimo-island>`;
+
+    await mod.initialize();
+    await mod.initialize();
+
+    expect(mockInitializeIslands).toHaveBeenCalledOnce();
+    expect(mockInitializeApps).toHaveBeenCalledTimes(2);
+    expect(
+      document.querySelector("marimo-island")?.classList.contains("marimo"),
+    ).toBe(true);
+  });
+
+  it("clears the memoized bootstrap when it fails so it can be retried", async () => {
+    const mod = await importMain();
+    document.body.innerHTML = `<marimo-island></marimo-island>`;
+    mockInitializeIslands.mockRejectedValueOnce(new Error("bootstrap failed"));
+
+    await expect(mod.initialize()).rejects.toThrow("bootstrap failed");
+    await mod.initialize();
+
+    expect(mockInitializeIslands).toHaveBeenCalledTimes(2);
+    expect(mockInitializeApps).toHaveBeenCalledOnce();
+  });
+});

frontend/src/core/islands/main.ts

@@ -20,7 +20,31 @@ import { parseMarimoIslandApps } from "./parse";
 const bridge = getGlobalBridge();
 let bootstrapPromise: Promise<void> | undefined;
 
-/** Returns whether the current document can replace its app in this worker. */
+/**
+ * Public entry point for the islands bundle.
+ *
+ * Islands auto-initialize on load. Hosts that keep the browser realm alive
+ * across client-side navigation (retaining this worker and its Pyodide
+ * environment) can drive page transitions with:
+ *
+ * ```ts
+ * if (canReplaceApp()) {
+ *   await stopApp();
+ *   updatePage();
+ *   await initialize();
+ * }
+ * ```
+ *
+ * The retained lifecycle (`canReplaceApp`/`stopApp`) only applies to pages
+ * whose islands resolve to a single app; multi-app pages are not replaceable.
+ */
+
+/**
+ * Returns whether the current document can replace its app in this worker.
+ *
+ * True only when the document contains exactly one replaceable app. This is a
+ * read-only probe: it does not materialize island payloads or mutate the DOM.
+ */
 export function canReplaceApp(): boolean {
   if (!document.querySelector(ISLAND_TAG_NAMES.ISLAND)) {
     return false;
@@ -30,6 +54,10 @@ export function canReplaceApp(): boolean {
 
 /**
  * Mounts marimo custom elements and starts the apps in the current document.
+ *
+ * Safe to call repeatedly, including after client-side navigation: the worker
+ * bootstrap is memoized (and cleared if it fails, so a later call retries),
+ * while app discovery re-runs on every call to pick up the current DOM.
  */
 export async function initialize(): Promise<void> {
   const islands = document.querySelectorAll<HTMLElement>(
@@ -49,7 +77,13 @@ export async function initialize(): Promise<void> {
   await bridge.initializeApps();
 }
 
-/** Stops the matching active app session. */
+/**
+ * Stops the matching active app session.
+ *
+ * With no `appId`, stops the current retained single-app session; with an
+ * `appId`, stops it only if it matches the active app. No-op when there is no
+ * retained session to stop.
+ */
 export async function stopApp(appId?: string): Promise<void> {
   await bridge.stopSession(appId);
 }