Attributes Edit

Block attributes provide information about the data stored by a block. For example, rich content, a list of image URLs, a background colour, or a button title.

A block can contain any number of attributes, and these are specified by the attributes field – an object where each key is the name of the attribute, and the value is the attribute definition.

The attribute definition will contain, at a minimum, either a type or an enum. There may be additional fields.

Example: Attributes object defining three attributes – url, title, and size.

{
    url: {
        type: 'string',
        source: 'attribute',
        selector: 'img',
        attribute: 'src',
    },
    title: {
        type: 'string',
    },
    size: {
        enum: [ 'large', 'small' ],
    },
}

When a block is parsed this definition will be used to extract data from the block content. Anything that matches will be available to your block through the attributes prop.

This parsing process can be summarized as:

  1. Extract value from the source.
  2. Check value matches the type, or is one of the enum values.

Example: Attributes available in the edit and function, using the above attributes definition.

function YourBlockEdit( { attributes } ) {
    return (
        <p>URL is { attributes.url }, title is { attributes.title }, and size is { attributes.size }.</p>
    )
}

The block is responsible for using the save function to ensure that all attributes with a source field are saved according to the attributes definition. This is not automatic.

Attributes without a source will be automatically saved in the block comment delimiter.

For example, using the above attributes definition you would need to ensure that your save function has a corresponding img tag for the url attribute. The title and size attributes will be saved in the comment delimiter.

Example: Example save function that contains the url attribute

function YourBlockSave( { attributes } ) {
    return (
        <img src={ attributes.url } />
    )
}

The saved HTML will contain the title and size in the comment delimiter, and the url in the img node.

<!-- block:your-block {"title":"hello world","size":"large"} -->
<img src="/image.jpg" />
<!-- /block:your-block -->

If an attributes change over time then a block deprecation can help migrate from an older attribute, or remove it entirely.

Type Validation

The type indicates the type of data that is stored by the attribute. It does not indicate where the data is stored, which is defined by the source field.

A type is required, unless an enum is provided. A type can be used with an enum.

The type field MUST be one of the following:

  • null
  • boolean
  • object
  • array
  • string
  • integer
  • number (same as integer)

Note that the validity of an object is determined by your source. For an example, see the query details below.

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Enum Validation

An attribute can be defined as one of a fixed set of values. This is specified by an enum, which contains an array of allowed values:

Example: Example enum.

{
    size: {
        enum: [ 'large', 'small', 'tiny' ]
    }
}

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Value Source

Attribute sources are used to define how the attribute values are extracted from saved post content. They provide a mechanism to map from the saved markup to a JavaScript representation of a block.

The available source values are:
(no value) – when no source is specified then data is stored in the block’s comment delimiter.
attribute – data is stored in an HTML element attribute.
text – data is stored in HTML text.
html – data is stored in HTML. This is typically used by RichText.
query – data is stored as an array of objects.
meta – data is stored in post meta (deprecated).

The source field is usually combined with a selector field. If no selector argument is specified, the source definition runs against the block’s root node. If a selector argument is specified, it will run against the matching element(s) within the block.

The selector can be an HTML tag, or anything queryable with querySelector, such as a class or id attribute. Examples are given below.

For example, a selector of img will match an img element, and img.class will match an img element that has a class of class.

Under the hood, attribute sources are a superset of the functionality provided by hpq, a small library used to parse and query HTML markup into an object shape.

To summarize, the source determines where data is stored in your content, and the type determines what that data is. To reduce the amount of data stored it is usually better to store as much data as possible within HTML rather than as attributes within the comment delimiter.

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attribute source

Use an attribute source to extract the value from an attribute in the markup. The attribute is specified by the attribute field, which must be supplied.

Example: Extract the src attribute from an image found in the block’s markup.

Saved content:

<div>
    Block Content

    <img src="https://lorempixel.com/1200/800/" />
</div>

Attribute definition:

{
    url: {
        type: 'string',
        source: 'attribute',
        selector: 'img',
        attribute: 'src',
    }
}

Attribute available in the block:

{ "url": "https://lorempixel.com/1200/800/" }

Most attributes from markup will be of type string. Numeric attributes in HTML are still stored as strings, and are not converted automatically.

Attribute definition:

{
    width: {
        type: 'string',
        source: 'attribute',
        selector: 'img',
        attribute: 'width',
    }
}

Attribute available in the block:

{ "width": "50" }

The only exception is when checking for the existence of an attribute (for example, the disabled attribute on a button). In that case type boolean can be used and the stored value will be a boolean.

Attribute definition:

{
    disabled: {
        type: 'boolean',
        source: 'attribute',
        selector: 'button',
        attribute: 'disabled',
    }
}

Attribute available in the block:

{ "disabled": true }

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text source

Use text to extract the inner text from markup. Note that HTML is returned according to the rules of textContent.

Saved content:

<figure>
    <img src="/image.jpg" />

    <figcaption>The inner text of the figcaption element</figcaption>
</figure>

Attribute definition:

{
    content: {
        type: 'string',
        source: 'text',
        selector: 'figcaption',
    }
}

Attribute available in the block:

{ "content": "The inner text of the figcaption element" }

Another example, using text as the source, and using .my-content class as the selector to extract text:

Saved content:

<div>
    <img src="/image.jpg" />

    <p class="my-content">The inner text of .my-content class</p>
</div>

Attribute definition:

{
    content: {
        type: 'string',
        source: 'text',
        selector: '.my-content',
    }
}

Attribute available in the block:

{ "content": "The inner text of .my-content class" }

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html source

Use html to extract the inner HTML from markup. Note that text is returned according to the rules of innerHTML.

Saved content:

<figure>
    <img src="/image.jpg" />

    <figcaption>The inner text of the <strong>figcaption</strong> element</figcaption>
</figure>

Attribute definition:

{
    content: {
        type: 'string',
        source: 'html',
        selector: 'figcaption',
    }
}

Attribute available in the block:

{ "content": "The inner text of the <strong>figcaption</strong> element" }

Use the multiline property to extract the inner HTML of matching tag names for the use in RichText with the multiline prop.

Attribute definition:

{
    content: {
        type: 'string',
        source: 'html',
        multiline: 'p',
        selector: 'blockquote',
    }
}

Attribute available in the block:

{ "content": "<p>First line</p><p>Second line</p>" }

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query source

Use query to extract an array of values from markup. Entries of the array are determined by the selector argument, where each matched element within the block will have an entry structured corresponding to the second argument, an object of attribute sources.

The query field is effectively a nested block attributes definition. It is possible (although not necessarily recommended) to nest further.

Example: Extract src and alt from each image element in the block’s markup.

Saved content:

<div>
    <img src="https://lorempixel.com/1200/800/" alt="large image" />
    <img src="https://lorempixel.com/50/50/" alt="small image" />
</div>

Attribute definition:

{
    images: {
        type: 'array',
        source: 'query',
        selector: 'img',
        query: {
            url: {
                type: 'string',
                source: 'attribute',
                attribute: 'src',
            },
            alt: {
                type: 'string',
                source: 'attribute',
                attribute: 'alt',
            },
        }
    }
}

Attribute available in the block:

{
    "images": [
        { "url": "https://lorempixel.com/1200/800/", "alt": "large image" },
        { "url": "https://lorempixel.com/50/50/", "alt": "small image" }
    ]
}

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Meta source (deprecated)

Although attributes may be obtained from a post’s meta, meta attribute sources are considered deprecated; EntityProvider and related hook APIs should be used instead, as shown in the Create Meta Block how-to.

Attributes may be obtained from a post’s meta rather than from the block’s representation in saved post content. For this, an attribute is required to specify its corresponding meta key under the meta key.

Attribute definition:

{
    author: {
        type: 'string',
        source: 'meta',
        meta: 'author'
    },
},

From here, meta attributes can be read and written by a block using the same interface as any attribute:

edit( { attributes, setAttributes } ) {
    function onChange( event ) {
        setAttributes( { author: event.target.value } );
    }

    return <input value={ attributes.author } onChange={ onChange } type="text" />;
},
edit: function( props ) {
    function onChange( event ) {
        props.setAttributes( { author: event.target.value } );
    }

    return el( 'input', {
        value: props.attributes.author,
        onChange: onChange,
    } );
},

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Considerations

By default, a meta field will be excluded from a post object’s meta. This can be circumvented by explicitly making the field visible:

function gutenberg_my_block_init() {
    register_post_meta( 'post', 'author', array(
        'show_in_rest' => true,
    ) );
}
add_action( 'init', 'gutenberg_my_block_init' );

Furthermore, be aware that WordPress defaults to:

  • not treating a meta datum as being unique, instead returning an array of values;
  • treating that datum as a string.

If either behavior is not desired, the same register_post_meta call can be complemented with the single and/or type parameters as follows:

function gutenberg_my_block_init() {
    register_post_meta( 'post', 'author_count', array(
        'show_in_rest' => true,
        'single' => true,
        'type' => 'integer',
    ) );
}
add_action( 'init', 'gutenberg_my_block_init' );

If you’d like to use an object or an array in an attribute, you can register a string attribute type and use JSON as the intermediary. Serialize the structured data to JSON prior to saving, and then deserialize the JSON string on the server. Keep in mind that you’re responsible for the integrity of the data; make sure to properly sanitize, accommodate missing data, etc.

Lastly, make sure that you respect the data’s type when setting attributes, as the framework does not automatically perform type casting of meta. Incorrect typing in block attributes will result in a post remaining dirty even after saving (cf. isEditedPostDirty, hasEditedAttributes). For instance, if authorCount is an integer, remember that event handlers may pass a different kind of data, thus the value should be cast explicitly:

function onChange( event ) {
    props.setAttributes( { authorCount: Number( event.target.value ) } );
}

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Default Value

A block attribute can contain a default value, which will be used if the type and source do not match anything within the block content.

The value is provided by the default field, and the value should match the expected format of the attribute.

Example: Example default values.

{
    type: 'string',
    default: 'hello world'
}
{
    type: 'array',
    default: [
        { "url": "https://lorempixel.com/1200/800/", "alt": "large image" },
        { "url": "https://lorempixel.com/50/50/", "alt": "small image" }
    ]
}
{
    type: 'object',
    default: {
        width: 100,
        title: 'title'
    }
}