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# Data Binding

Connect UI elements to dynamic data using expressions in your JSON specs.

## State Model

Every spec can include a `state` object that holds the data your UI reads from:

```json
{
  "root": "greeting",
  "elements": {
    "greeting": {
      "type": "Text",
      "props": { "content": { "$state": "/user/name" } },
      "children": []
    }
  },
  "state": {
    "user": { "name": "Alice" }
  }
}
```

State can also be provided programmatically at runtime. In `@json-render/react`, this is done via `StateProvider` and hooks like `useStateStore`. See the [React API reference](/docs/api/react) for details.

## JSON Pointer Paths

All paths in json-render follow JSON Pointer (RFC 6901). A path is a string of `/`-separated tokens starting from the root:

```
Given this state:
{
  "user": { "name": "Alice", "email": "alice@example.com" },
  "todos": [
    { "title": "Buy milk", "done": false },
    { "title": "Walk dog", "done": true }
  ]
}

"/user/name"      -> "Alice"
"/user/email"     -> "alice@example.com"
"/todos/0/title"  -> "Buy milk"
"/todos/1/done"   -> true
```

## Expressions

Expressions are special objects you place in props to read dynamic values instead of hardcoding them. There are six expression types.

### `$state` — Read from state

Use `{ "$state": "/path" }` in any prop to read a value from the state model:

```json
{
  "type": "Card",
  "props": {
    "title": { "$state": "/user/name" },
    "subtitle": { "$state": "/user/email" }
  },
  "children": []
}
```

If state contains `{ "user": { "name": "Alice", "email": "alice@example.com" } }`, the Card renders with title "Alice" and subtitle "alice@example.com".

### `$item` — Read from the current repeat item

Use `{ "$item": "field" }` inside a [repeat](#repeat) to read a field from the current array item:

```json
{
  "type": "Text",
  "props": { "content": { "$item": "title" } },
  "children": []
}
```

Use `{ "$item": "" }` to get the entire item object.

### `$index` — Current repeat index

Use `{ "$index": true }` inside a [repeat](#repeat) to get the current array index (zero-based number):

```json
{
  "type": "Text",
  "props": { "content": { "$index": true } },
  "children": []
}
```

## Repeat

The `repeat` field on an element renders its children once per item in a state array. It is a top-level field on the element, sibling of `type`, `props`, and `children` — not inside `props`.

```json
{
  "root": "todo-list",
  "elements": {
    "todo-list": {
      "type": "Column",
      "props": { "gap": 8 },
      "repeat": { "statePath": "/todos", "key": "id" },
      "children": ["todo-item"]
    },
    "todo-item": {
      "type": "Card",
      "props": {
        "title": { "$item": "title" },
        "subtitle": { "$item": "description" }
      },
      "children": []
    }
  },
  "state": {
    "todos": [
      { "id": "1", "title": "Buy milk", "description": "2% or whole" },
      { "id": "2", "title": "Walk dog", "description": "Around the park" }
    ]
  }
}
```

- `repeat.statePath` — JSON Pointer to the state array
- `repeat.key` — field name on each item to use as a stable key for rendering

Inside `todo-item`, `{ "$item": "title" }` reads the `title` field from whichever array item is currently being rendered. `{ "$index": true }` would return `0` for the first item, `1` for the second, and so on.

## Two-Way Binding with `$bindState`

Form components use `{ "$bindState": "/path" }` on their natural value prop for two-way binding. The component reads from and writes to the state path.

### Value prop (text inputs)

```json
{
  "type": "TextInput",
  "props": {
    "value": { "$bindState": "/form/email" },
    "placeholder": "Enter your email"
  },
  "children": []
}
```

### Checked prop (switches, checkboxes)

```json
{
  "type": "Switch",
  "props": {
    "label": "Enable notifications",
    "checked": { "$bindState": "/settings/notifications" }
  },
  "children": []
}
```

### Pressed prop (toggle buttons)

```json
{
  "type": "ToggleButton",
  "props": {
    "label": "Bold",
    "pressed": { "$bindState": "/editor/bold" }
  },
  "children": []
}
```

## Two-Way Binding with `$bindItem`

Inside a repeat scope, use `{ "$bindItem": "field" }` to bind to a field on the current item:

```json
{
  "type": "Switch",
  "props": {
    "label": "Done",
    "checked": { "$bindItem": "completed" }
  },
  "children": []
}
```

Use `{ "$bindItem": "" }` to bind to the entire item.

`statePath` is not used for component binding. It remains for `repeat.statePath` (array iteration path) and action params like `setState.statePath` (target path for mutations).

## Conditional Props

Use `$cond` / `$then` / `$else` to pick a prop value based on a condition:

```json
{
  "type": "Badge",
  "props": {
    "label": {
      "$cond": { "$state": "/user/isAdmin" },
      "$then": "Admin",
      "$else": "Member"
    }
  },
  "children": []
}
```

The condition uses the same [visibility](/docs/visibility) expression format.

## Quick Reference

<div className="my-6 overflow-x-auto">
  <table className="mdx-table w-full text-sm border-collapse">
    <thead>
      <tr>
        <th>Expression</th>
        <th>Syntax</th>
        <th>Context</th>
      </tr>
    </thead>
    <tbody>
      <tr>
        <td><code>{"$state"}</code></td>
        <td><code>{'{ "$state": "/path" }'}</code></td>
        <td>Anywhere</td>
      </tr>
      <tr>
        <td><code>{"$item"}</code></td>
        <td><code>{'{ "$item": "field" }'}</code></td>
        <td>Inside repeat only</td>
      </tr>
      <tr>
        <td><code>{"$index"}</code></td>
        <td><code>{'{ "$index": true }'}</code></td>
        <td>Inside repeat only</td>
      </tr>
      <tr>
        <td><code>{"$cond"}</code></td>
        <td><code>{'{ "$cond": ..., "$then": ..., "$else": ... }'}</code></td>
        <td>Anywhere</td>
      </tr>
      <tr>
        <td><code>{"$bindState"}</code></td>
        <td><code>{'{ "$bindState": "/path" }'}</code></td>
        <td>Form components (value, checked, pressed)</td>
      </tr>
      <tr>
        <td><code>{"$bindItem"}</code></td>
        <td><code>{'{ "$bindItem": "field" }'}</code></td>
        <td>Form components inside repeat</td>
      </tr>
    </tbody>
  </table>
</div>

## Next

- [Visibility](/docs/visibility) — conditionally show or hide elements
- [Action handlers](/docs/registry#action-handlers) — respond to user interactions
- [React API reference](/docs/api/react) — React-specific hooks for programmatic state access
