# ViewData (Website Templates)

ViewData is the **only** allowed input type for Templates in `apps/website`.

Authoritative contract: [`WEBSITE_CONTRACT.md`](docs/architecture/website/WEBSITE_CONTRACT.md:1).

## 1) Definition

ViewData is a JSON-serializable, template-ready data structure:

- primitives (strings/numbers/booleans)
- arrays and plain objects
- `null` for missing values

## 2) What ViewData is NOT

ViewData is not:

- an API Transport DTO (raw transport)
- a ViewModel (client-only class)
- a Display Object instance

## 3) Construction rules

ViewData is created by **ViewData Builders**:

### Server Components (RSC)
```typescript
const apiDto = await PageQuery.execute();
const viewData = ViewDataBuilder.build(apiDto);
return <Template viewData={viewData} />;
```

### Client Components
```typescript
'use client';

const [viewModel, setViewModel] = useState<ViewModel | null>(null);

useEffect(() => {
  const apiDto = await apiClient.get();
  const vm = ViewModelBuilder.build(apiDto);
  setViewModel(vm);
}, []);

// Template receives ViewData from ViewModel
return viewModel ? <Template viewData={viewModel.viewData} /> : null;
```

Templates MUST NOT compute derived values.

ViewData Builders MUST NOT call the API.

## 4) Formatting and SEO

ViewData is responsible for providing **fully formatted strings** to Templates for Server-Side Rendering (SSR).

- **SEO Requirement:** All data required for search engines (prices, dates, counts, labels) MUST be formatted in the `ViewData Builder` on the server.
- **Template Simplicity:** Templates should simply render the strings provided in `ViewData` without further processing.

## 5) Determinism rules

Any formatting used to produce ViewData MUST be deterministic.

Forbidden anywhere in formatting code paths:

- `Intl.*`
- `Date.toLocaleString()` / `Date.toLocaleDateString()` / `Date.toLocaleTimeString()`

Reason: SSR and browser outputs can differ.

Localization MUST NOT depend on runtime locale APIs.
If localized strings are required, they MUST be provided as deterministic inputs (for example via API-provided labels or a deterministic code-to-label map) and passed through ViewData Builders into ViewData.

## 5) Relationship to Display Objects

Display Objects are used to implement formatting/mapping, but their instances MUST NOT be stored inside ViewData.

Only primitive outputs produced by Display Objects may appear in ViewData.
