# Computed Values

Derive dynamic prop values using registered functions or string templates.

## `$template` — String Interpolation

Use `{ "$template": "..." }` to embed state values into a string. References use `${/path}` syntax where the path is a JSON Pointer:

```json
{
  "type": "Text",
  "props": {
    "text": { "$template": "Hello, ${/user/name}! You have ${/inbox/count} messages." }
  },
  "children": []
}
```

If state is `{ "user": { "name": "Alice" }, "inbox": { "count": 3 } }`, the text renders as "Hello, Alice! You have 3 messages."

Missing paths resolve to an empty string.

## `$computed` — Registered Functions

Use `{ "$computed": "<name>", "args": { ... } }` to call a named function registered in your catalog. Each arg can be a literal value or any prop expression (`$state`, `$item`, `$cond`, etc.):

```json
{
  "type": "Text",
  "props": {
    "text": {
      "$computed": "fullName",
      "args": {
        "first": { "$state": "/form/firstName" },
        "last": { "$state": "/form/lastName" }
      }
    }
  },
  "children": []
}
```

### Registering Functions

Functions are registered in the catalog and provided at runtime.

**Catalog definition (for AI prompt generation):**

```typescript

const catalog = defineCatalog(schema, {
  components: { /* ... */ },
  functions: {
    fullName: {
      description: 'Combines first and last name into a full name',
    },
    formatCurrency: {
      description: 'Formats a number as currency',
    },
  },
});
```

**Runtime implementation:**

```tsx

const functions = {
  fullName: (args) => `${args.first ?? ''} ${args.last ?? ''}`.trim(),
  formatCurrency: (args) => {
    const value = Number(args.value ?? 0);
    return new Intl.NumberFormat('en-US', {
      style: 'currency',
      currency: (args.currency as string) ?? 'USD',
    }).format(value);
  },
};

<JSONUIProvider registry={registry} functions={functions}>
  <Renderer spec={spec} registry={registry} />
</JSONUIProvider>
```

### Using with `createRenderer`

```tsx
const MyRenderer = createRenderer(catalog, components);

<MyRenderer
  spec={spec}
  functions={functions}
/>
```

## Combining Expressions

`$computed` args can use any expression type. This example computes a total from repeat item fields:

```json
{
  "$computed": "lineTotal",
  "args": {
    "price": { "$item": "price" },
    "quantity": { "$item": "quantity" }
  }
}
```

## Next

- [Watchers](/docs/watchers) — react to state changes with cascading actions
- [Data Binding](/docs/data-binding) — all expression types
- [Validation](/docs/validation) — validate form inputs