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# Reactivity

> **Studio writes this format for you.** The **Data** panel and the Inspector's **Logic** tab ([Logic](/docs/studio/logic)) generate everything below. This page documents the model if you want to hand-edit or understand it.

Template literal syntax ${} is valid **anywhere a string value appears** in the document tree, not only inside `state`. Wherever you write one, the value recomputes when the state it reads changes. All reactivity is powered by `@vue/reactivity`.

A template resolves `state.propertyName` against the current component's reactive proxy, plus the iteration bindings (`$map`, `item`, `index`) where an Array map provides them.

## Reactive element properties

```json shiki
```

Note

**A binding that throws waits for its data.**
 State that loads asynchronously reads 
`null`
 first, so 
`${state.posts.length}`
 throws until the posts arrive. The binding renders nothing in the meantime, logs one console warning, and fills in on its own once the value it reads is valid.

## Reactive style properties

```json shiki
```

## Reactive attributes

```json shiki
```

## How it works

When the compiler encounters ${} in any string-valued property, it wraps the binding in a reactive effect:

```js shiki
```

Dependencies are tracked automatically by Vue when `state.*` properties are read.

## Choosing between `$ref` and a template string

Prefer ${} for a single-use reactive binding, and `$ref` for a signal that is named and reused.

| Pattern                       | Use when                                          |
| ----------------------------- | ------------------------------------------------- |
| `{ "$ref": "#/state/label" }` | Binding to a named signal used in multiple places |
| "${state.count} items"        | Inline computed binding used in exactly one place |

## Computed state

Template strings in `state` become `computed()` values:

```json shiki
```

## Reading and writing state from JavaScript

Within `body` strings and external `.js` files, read and write state directly:

```js shiki
```

There are no `.get()` or `.set()` calls and no `this`. All component state is reached through `state`.

## Prototypes for web APIs

Built-in prototypes for common web APIs:

| `$prototype`      | Web API      | Description                   |
| ----------------- | ------------ | ----------------------------- |
| `Request`         | Fetch API    | Reactive URL, debounce, abort |
| `URLSearchParams` | URL API      | Computed `.toString()`        |
| `FormData`        | FormData API | Field population              |
| `LocalStorage`    | Storage API  | Reactive persistence          |
| `SessionStorage`  | Storage API  | Session-scoped storage        |
| `IndexedDB`       | IDB API      | Store creation, CRUD          |
| `Array`           | —            | Dynamic mapped lists          |

## Timing

| Value        | When                                            |
| ------------ | ----------------------------------------------- |
| `"client"`   | Resolved at runtime in the browser (default)    |
| `"server"`   | Resolved at runtime on the server via RPC       |
| `"compiler"` | Resolved at build time, baked into emitted HTML |

## Related

- [State](/docs/framework/concepts/state) declares the signals a template reads.
- [Expressions](/docs/framework/concepts/expressions) covers what may appear inside ${}.
- [Timing](/docs/framework/concepts/timing) covers when each value resolves.

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