storybook-play-functions

name	storybook-play-functions
description	Storybook `play` functions in snapmatch — interaction tests authored INSIDE the story using `@storybook/test` (`expect`, `userEvent`, `within`), executed inside Storybook's iframe at story render. Distinct from external Playwright story tests. Triggers on: play function, @storybook/test, userEvent, interaction test, story interaction, play step.
license	MIT

Sub-skill of storybook. Owns the play function field on StoryObj — the interactions that run inside Storybook's iframe at story render time, using @storybook/test's expect, userEvent, and within (or the destructured canvas / userEvent / step from the play context). Plays are different from Playwright story tests: a play runs as part of the story (and is what an external Playwright test typically exercises); a Playwright story test runs outside the iframe and asserts on whatever the play produced.

When to invoke

• Adding interaction(s) to a *.stories.tsx (typing, clicking, hovering, asserting on visible state).
• Asserting that an args.onSomething callback was called (via fn() from @storybook/test).
• Grouping interactions into named steps (step('...', async () => {...})).
• Choosing between an in-story play function and an external Playwright story test (see the decision section).
• Diagnosing a play function that passes interactively but fails when Playwright drives the same URL.

Owns

play functions, @storybook/test (userEvent, expect), interaction testing inside the story — the surface Playwright drives.

Defers to

• storybook (parent) — version pin and routing.
• storybook-stories — for the Meta/StoryObj shape the play function attaches to (play is a field on the StoryObj).
• storybook-config — for @storybook/test install + the global decorators that wrap the iframe.
• playwright-story-tests — the decision of "play function vs Playwright test" lives here; the canonical decision rule is at the bottom of this skill, but the Playwright sub-skill owns the external-spec authoring.
• playwright-conventions — for the ASK-FIRST rule that governs any Playwright test design (a play function does NOT require ASK-FIRST; only Playwright tests do).
• react (router) — for the component patterns the play function exercises.
• idb — when an interaction's outcome is "an IDB record was written"; the play can await, but assertions on real IDB content belong in Playwright (see decision).

Snapmatch stack rules

• Pillar 1 (Storybook-first) means: a play function lives next to the component it exercises. The play is part of the story file and serves both interactive debugging and Playwright story tests.
• Pillar 4 (CLI-gate-first) means: a play function that throws causes the story to render in an error state — caught by Playwright story tests in the gate. A red play in dev (bun run storybook) is a failed contract; fix it before continuing.
• @storybook/test is the only allowed source of userEvent / expect / within inside a story. Never @storybook/testing-library (deprecated, removed in 8.x). Never @testing-library/user-event directly.
• Mock callbacks with fn() from @storybook/test, not bare vi.fn() / jest.fn(). The fn() mocks show up in the Interactions panel; raw vi.fn() does not.
• Plays run inside the Storybook iframe — they have access to the rendered DOM and can await realistic timers, but they do not have access to Playwright's network mocking, offline mode, or cross-page navigation.

Patterns

Minimal play function — assert on args.onClick

// apps/example-game/app/components/button/index.stories.tsx (excerpt)
// pinned: storybook ^10.x, @storybook/test ^10.x
import type { Meta, StoryObj } from "@storybook/react-vite";
import { expect, fn, userEvent, within } from "@storybook/test";
import { Button } from "./index";

const meta = {
  title: "Components/Button",
  component: Button,
  // `fn()` from @storybook/test produces a mock that shows in the Interactions panel.
  args: { onClick: fn() },
} satisfies Meta<typeof Button>;
export default meta;
type Story = StoryObj<typeof meta>;

export const ClicksFire: Story = {
  args: { label: "Click me" },
  play: async ({ args, canvasElement }) => {
    const canvas = within(canvasElement);
    await userEvent.click(canvas.getByRole("button", { name: /click me/i }));
    await expect(args.onClick).toHaveBeenCalledOnce();
  },
};

Use within(canvasElement) to scope queries to the rendered story (not to the entire iframe, which includes Storybook's chrome). expect(args.onClick).toHaveBeenCalledOnce() works because args.onClick is the fn() mock declared at the meta level.

Step grouping — readable plays

export const Submits: Story = {
  args: { onSubmit: fn() },
  play: async ({ args, canvasElement, step }) => {
    const canvas = within(canvasElement);
    await step("Fill credentials", async () => {
      await userEvent.type(canvas.getByLabelText(/email/i), "[e-mail]");
      await userEvent.type(canvas.getByLabelText(/password/i), "hunter2");
    });
    await step("Submit", async () => {
      await userEvent.click(canvas.getByRole("button", { name: /sign in/i }));
      await expect(args.onSubmit).toHaveBeenCalledTimes(1);
    });
  },
};

Steps appear as collapsible groups in the Interactions panel and in Playwright traces (when a Playwright story test drives the same URL). Use them to make multi-step flows scannable; one step per logical phase.

Async waits — prefer findBy* over manual timeouts

play: async ({ canvasElement }) => {
  const canvas = within(canvasElement);
  await userEvent.click(canvas.getByRole("button", { name: /save/i }));
  // findByRole auto-retries until visible OR timeout — no fixed sleep.
  await expect(await canvas.findByRole("status")).toHaveTextContent(/saved/i);
};

findBy* retries until the element is found or the test timeout expires. Never insert a fixed setTimeout / waitForTimeout — it hides real flake.

Querying outside the canvas (portals, modals)

import { expect, screen, userEvent, within } from "@storybook/test";

play: async ({ canvasElement }) => {
  const canvas = within(canvasElement);
  await userEvent.click(canvas.getByRole("button", { name: /open dialog/i }));
  // The dialog renders to document.body via a portal — use `screen` to query it.
  await expect(screen.getByRole("dialog")).toBeVisible();
};

screen queries the entire document. Use for portals, toasts, tooltips that render outside the story's root.

fn() for callbacks — never bare jest/vi mocks

const meta = {
  args: {
    onSelect: fn(),  // CORRECT — appears in the Interactions panel
    // onSelect: vi.fn(),   // WRONG — invisible to the panel
    // onSelect: jest.fn(), // WRONG — invisible to the panel
  },
} satisfies Meta<typeof MyComponent>;

Step → Playwright trace alignment

play: async ({ step, canvasElement }) => {
  await step("hydrated", async () => {
    const canvas = within(canvasElement);
    await canvas.findByRole("main"); // hydration completed; the rest is post-hydration
  });
};

Naming a step "hydrated" gives the external Playwright story test (see playwright-story-tests) a stable anchor in the trace — handy when an iPad-over-LAN flake report needs to know whether hydration completed before the next interaction.

Decision — play function vs external Playwright story test

Use a play function when:

• The interaction logic is fully inside the story's DOM.
• The assertion is "callback was called," "visible state changed," or "ARIA state updated."
• You want the assertion to run interactively in bun run storybook for inner-loop feedback.

Use an external Playwright story test (see playwright-story-tests) when:

• The assertion needs real IDB content (page.evaluate(() => indexedDB...) to read records).
• The assertion needs network mocking, offline mode, service-worker behavior — none of which exist inside the iframe.
• The assertion needs cross-iframe / cross-page state.
• The story has a play and you want to verify its post-conditions externally (the play is the setup; Playwright is the assertion).

The two are not exclusive: a story can have a play (for in-iframe interactions and inner-loop feedback) and an external Playwright test (for IDB / network / offline assertions). When you'd write a Playwright test, ASK FIRST per Pillar 4 — the user owns the structural decisions for every Playwright test (see playwright-conventions). No ASK-FIRST is needed to add or modify a play function — it's part of the story.

Install

bun add -D @storybook/test@^10

Pin to the same Storybook 10 majors as @storybook/react-vite (see storybook-config). Do not install @storybook/testing-library — removed in 8.x, gone in 10.

Anti-patterns

• Don't import from @storybook/testing-library — deprecated, removed. Use @storybook/test for userEvent, expect, within, screen, fn.
• Don't import userEvent from @testing-library/user-event — use the re-export from @storybook/test so the Interactions panel sees the calls.
• Don't use bare vi.fn() / jest.fn() for callback args — use fn() from @storybook/test. Bare mocks don't show in the Interactions panel.
• Don't insert setTimeout / waitForTimeout to "wait for state" — use findBy* (auto-retry) or await expect(locator).toBe…() semantics. Fixed sleeps mask real flake.
• Don't query the whole iframe with document.querySelector — scope with within(canvasElement). Querying outside the canvas hits Storybook's chrome and breaks isolation.
• Don't put a Playwright expect inside a play function — @storybook/test's expect is the right one (it is jest-compatible and ships with Storybook). Playwright's web-first expect only works inside Playwright specs.
• Don't reach for IDB assertions inside a play function — assertions on indexedDB belong in Playwright (see playwright-story-tests). A play can trigger IDB writes; verifying them is Playwright's job.
• Don't ask the user for permission to write a play function — the ASK-FIRST rule is Playwright-only. Plays are part of the story authoring surface.
• Don't write a play function that depends on network calls — there's no network mocking inside the iframe. If the component needs network, mock at the args level or push the assertion to Playwright.

Triggers on

play function, @storybook/test, userEvent, interaction test, story interaction, play step