atomic-design-poc/libs/shared/docs/testing.mdx

import { Meta } from '@storybook/addon-docs/blocks';

<Meta title="Foundations/Testing strategy" />

# Testing strategy

Tests follow the same grain as the architecture: **push the logic down to where it's pure,
test it there directly, and keep the layers above thin.** No single tool covers everything,
so each layer gets the cheapest test that catches its class of bug. This page owns _what to
test, by layer_; how each test is **phrased and scoped** — one behaviour, in the domain's
language — is [BDD](?path=/docs/foundations-bdd--docs).

## What gets tested where

| Layer                      | Test kind                | Tool             | Rule                                                                                                                   |
| -------------------------- | ------------------------ | ---------------- | ---------------------------------------------------------------------------------------------------------------------- |
| `domain/`                  | pure-function unit spec  | Vitest           | **Required.** Call the function directly — **no `TestBed`**. Reducers, combinators, `visibleSteps`, parsers, policies. |
| `domain/value-objects/`    | parser spec              | Vitest           | Happy path + normalisation + **each** rejection. Assert on the `Result`, never the message.                            |
| `infrastructure/` `parse*` | trust-boundary spec      | Vitest           | Accept a valid DTO; **reject `null` / `{}` / malformed**. Name it `describe('… (trust boundary)')`.                    |
| `application/`             | store / command spec     | Vitest           | Reducer purity, optimistic begin→confirm/rollback, command `Result`.                                                   |
| `ui/`                      | Storybook story          | Storybook + a11y | Kept thin. Axe runs on every story; add a `play` only for wiring axe can't see.                                        |
| flows                      | e2e smoke                | Playwright       | One happy path + one error state per critical journey.                                                                 |
| backend                    | rule + endpoint + golden | xUnit            | Mirror of the FE domain rules, plus `WebApplicationFactory` integration.                                               |

## Tooling

Vitest runs through Angular's built-in `@angular/build:unit-test` builder — **there is no
`vitest.config.ts`**; config is implicit via `tsconfig.spec.json`.

```bash
npm test           # ng test → Vitest, all *.spec.ts co-located next to their unit
```

Specs import `{ describe, it, expect }` from `vitest` and are co-located with the unit
they cover.

## The house style

Pure and direct. A value-object parser spec
(`registratie/domain/value-objects/postcode.spec.ts`):

```ts
import { describe, it, expect } from 'vitest';
import { parsePostcode } from './postcode';

describe('parsePostcode', () => {
  it('normalises to "1234 AB"', () => {
    const r = parsePostcode(' 1234ab ');
    expect(r.ok).toBe(true);
    if (r.ok) expect(r.value).toBe('1234 AB');
  });
  it('rejects malformed input', () => {
    expect(parsePostcode('0234AB').ok).toBe(false); // asserts the tag, not the copy
  });
});
```

A trust-boundary adapter (`registratie/infrastructure/brp.adapter.spec.ts`) additionally
proves the untrusted shape is rejected:

```ts
expect(parseBrpAddress(null).ok).toBe(false);
expect(parseBrpAddress({}).ok).toBe(false); // missing required field
```

Elm-style machines test the pure `reduce` — no Angular
(`registratie/domain/registratie-wizard.machine.spec.ts`).

## Fixtures: build test data through the production door

A fixture is not a shortcut around the domain — it's the domain's own construction path, run
once for the test. [ADR-0006](../../../docs/reference/architecture/0006-test-data-builders.md)
covers this in full (with a backend example too); the frontend idiom is one combinator,
`given` (`libs/shared/src/testing/machine.ts`):

```ts
export const given =
  <S, M>(reduce: (s: S, m: M) => S, initial: S) =>
  (...msgs: M[]): S =>
    msgs.reduce(reduce, initial);

export const givenIntake = given(reduce, initial); // per-context wrapper, pure TS
```

A machine spec replays real `Msg`s instead of hand-writing a `State` literal — so a fixture
can only ever be a state the real reducer actually produces:

```ts
const atStep3 = givenIntake(Start(), SetUren('1200'), Next(), SetDiplomaHerkomst('NL'), Next());
```

The same rule extends to value objects — call the real `parse*` and check `.ok` before use,
never a cast — and to `RemoteData` (`loading()` / `success(v)` / `failure(e)` in
`libs/shared/src/testing/remote-data.ts` instead of a redefined-per-file literal). **Never**
a `.withX().withY()` builder over an open constructor — that just re-opens whatever illegal
state the domain closed.

## UI = Storybook, not heavy component tests

`@storybook/addon-a11y` runs the `wcag2a/2aa/21a/21aa` rule sets on **every** story;
`@storybook/test-runner` + `axe-playwright` turn that into a CI gate:

```bash
npm run test-storybook       # axe over every story against a running Storybook
npm run test-storybook:ci    # builds storybook-static, serves :6006, runs the gate
```

Disabling a11y on a story needs an inline justification + a WP cross-reference (see
[Accessibility](?path=/docs/foundations-accessibility--docs)).

## Don't assert on copy

Localized strings change per locale and per edit. Tests assert on the `Result`
discriminant, the value object, or the message **id** — never the rendered Dutch/English
text. Full detail in [Internationalization](?path=/docs/foundations-internationalization--docs).

## The GREEN gate

```bash
npm run lint && npm run check:tokens && npm test && npm run build && npm run build-storybook
cd backend && dotnet test
```

Everything above must pass before a work package is done.