---
title: "lzb/prepare_block_attribute"
description: "PHP filter `lzb/prepare_block_attribute` of the Lazy Blocks WordPress plugin."
url: "https://www.lazyblocks.com/docs/php-filters/lzb-prepare_block_attribute/"
lastUpdated: 2026-09-09
source: "php-filters/lzb-prepare_block_attribute.mdx"
---
# lzb/prepare_block_attribute

Sets the storage type and default of the Gutenberg attribute behind one control, which a custom control needs when its value is not a plain string.

It runs once per control while the attribute list for `register_block_type()` is built, after [lzb/block_data](https://www.lazyblocks.com/docs/php-filters/lzb-block_data/) has assembled the block and before [lzb/register_block_type_data](https://www.lazyblocks.com/docs/php-filters/lzb-register_block_type_data/) sees the finished registration array. Two loops fire it. `prepare_block_attributes()` handles the controls stored in the block markup, and `prepare_block_meta_attributes()` handles the ones with **Save in Meta** on, whose `$attribute_data` carries the extra `source` and `meta` keys.

## Attributes

| Name              | Type       | Description                                                                            |
| ----------------- | ---------- | -------------------------------------------------------------------------------------- |
| `$attribute_data` | **Array**  | `type` and `default`, plus `source` and `meta` for a meta control                      |
| `$control`        | **Array**  | this control, merged with its type's defaults                                          |
| `$controls`       | **Array**  | every control on the block, keyed by control ID                                        |
| `$k`              | **String** | control ID of `$control` inside `$controls`                                            |
| `$block`          | **Array**  | block data, the same array [lzb/block_data](https://www.lazyblocks.com/docs/php-filters/lzb-block_data/) returns |

`$attribute_data['type']` comes from the control type's own `type` property and is one of `string`, `number`, `boolean`, `array` or `object`. `$attribute_data['default']` is the control's `default` value, cast to a float when the type is `number` and the default is not an empty string.

`$control` always has `type`, `name`, `default`, `label`, `help`, `child_of`, `placement`, `group`, `width`, `hide_if_not_selected`, `required`, `translate`, `save_in_meta` and `save_in_meta_name`, because the type's defaults are merged in first. Controls are visited only when their `child_of` matches the level being built, so a Repeater's inner controls arrive on their own pass.

## Usage

```php title="PHP"
function my_lzb_prepare_block_attribute( $attribute_data, $control, $controls, $k, $block ) {
  // A custom control stores JSON, so register it as an object rather than
  // the string its control class declares.
  if ( 'my-json-control' !== $control['type'] ) {
    return $attribute_data;
  }

  $attribute_data['type']    = 'object';
  $attribute_data['default'] = array( 'mode' => 'compact' );

  return $attribute_data;
}

add_filter( 'lzb/prepare_block_attribute', 'my_lzb_prepare_block_attribute', 10, 5 );
```

The type declared here is what Gutenberg validates the saved value against, so a mismatch makes the editor report the block as invalid after a reload rather than failing at save time. For a **Save in Meta** control the same array is also handed to `register_meta()`. Setting `type` to `array` without also setting `items` makes Lazy Blocks fall back to `show_in_rest => true`, and WordPress refuses to expose array meta over REST without an items schema. Dropping the `default` key gives the attribute a `null` default, which is different from an empty string and shows up in Handlebars as nothing at all.

To change the value at render time instead of the type at registration time, use [lzb/control_value](https://www.lazyblocks.com/docs/php-filters/lzb-control_value/).

## Documentation Index
> Fetch the complete documentation index at: https://www.lazyblocks.com/llms.txt
> Fetch every page in a single file at: https://www.lazyblocks.com/llms-full.txt