storybook

name	storybook
description	Storybook conventions — CSF3 stories, play functions as Vitest component tests, decorators, a11y. Use when writing or editing *.stories.tsx or Storybook config, AND whenever asked to write tests for a component — stories with play functions are this repo's component-test layer. 日本語の依頼例:「Storybook/ストーリー書いて」「コンポーネントのテスト書いて」「play関数」「interaction test」。

Storybook

Stories are the component-test layer: every story renders in the UI catalog AND runs as a Vitest browser-mode test via the Storybook Vitest addon. Write them as both documentation and test.

CSF3 shape

import type { Meta, StoryObj } from '@storybook/react'; // or framework package per installed SB version
import { expect, fn, userEvent, within } from 'storybook/test';
import { LoginForm } from './LoginForm';

const meta = {
  component: LoginForm,
  args: { onSubmit: fn() },          // fn() for every callback prop — assertable, visible in Actions panel
} satisfies Meta<typeof LoginForm>;
export default meta;

type Story = StoryObj<typeof meta>;

• Colocate: LoginForm.tsx + LoginForm.stories.tsx. satisfies Meta<…> + StoryObj<typeof meta> — full inference, no casts.
• Check installed Storybook major version before using version-specific imports (storybook/test vs @storybook/test).

Which stories to write

• One story per meaningful state: default, empty, loading, error, edge content (long text, many items). Each renders without interaction → these are your visual states, and the VRT snapshot surface (see visual-regression).
• Plus interaction stories with play for each key behavior: submit success, validation error, keyboard operation.
• Vary stories via args, not copy-pasted render functions. Custom render only when composition is needed.

Play functions

export const ShowsValidationError: Story = {
  play: async ({ canvas, step, args }) => {
    await step('submit empty form', async () => {
      await userEvent.click(canvas.getByRole('button', { name: /log in/i }));
    });
    await step('shows error, does not submit', async () => {
      await expect(canvas.getByRole('alert')).toHaveTextContent(/required/i);
      await expect(args.onSubmit).not.toHaveBeenCalled();
    });
  },
};

• await every interaction and assertion — a missing await passes locally and flakes in CI.
• Same query rules as Testing Library: getByRole first (see testing-vitest).
• step() for multi-phase plays — failures pinpoint the phase.
• Assert outcomes: error visible AND callback not called; not just "no crash".
• Portals/modals render outside the canvas — query via within(canvasElement.parentElement!) or screen-level helpers.

Decorators & data

• Global providers (theme, i18n, MemoryRouter) once in .storybook/preview.tsx decorators — never per-story copies.
• Network-dependent components: MSW (msw-storybook-addon), sharing handler definitions with Vitest tests. No fetch stubbing inside stories.
• A story requiring 30 lines of setup is telling you the component's dependencies are too broad.

Running as tests

• The Vitest addon turns every story into a test (render check) and runs play functions in a real browser (Playwright provider) — keep npm run test covering them in CI.
• Pin the React runtime (and any animation lib) in the Storybook test project's optimizeDeps.include: react, react-dom, react-dom/client, react/jsx-runtime, react/jsx-dev-runtime, plus e.g. motion/react. Otherwise Vite discovers a dep mid-run, re-optimizes, and reloads the page — the play function then queries an unmounted tree: an intermittent "unable to find element" that often only shows in CI. The [vitest] Vite unexpectedly reloaded a test warning is the tell.
• A story that can't pass headlessly (depends on viewport quirks, real network) is broken — fix the story, don't exclude it.
• Plays that follow an animation (modal close, exit transitions) assert post-animation state with auto-retrying queries (findBy*, retried expect) — do NOT disable animations or emulate reduced motion in interaction tests; that executes a different motion code path than users get. Animations-off belongs to VRT only (see visual-regression).
• Reuse stories in plain Vitest tests with composeStories when you need extra assertions beyond the play function.

A11y

• Keep @storybook/addon-a11y enabled with serious/critical violations failing the test run (same threshold as the Playwright axe gate), not just warnings in the panel. Fix or explicitly (with reason) disable specific rules per story — never globally.