datastar/site/static/md/reference/attribute_plugins.md

# Attribute Plugins

Datastar provides the following [`data-*`](https://developer.mozilla.org/en-US/docs/Learn/HTML/Howto/Use_data_attributes) attributes.

<div class="alert alert-info">
    <iconify-icon icon="simple-icons:rocket"></iconify-icon>
    <div>
        The Datastar <a href="https://marketplace.visualstudio.com/items?itemName=starfederation.datastar-vscode">VSCode extension</a> and <a href="https://plugins.jetbrains.com/plugin/26072-datastar-support">IntelliJ plugin</a> provided autocompletion for all <code>data-*</code> attributes.
    </div>
</div>

### Attribute Order

Note that `data-*` attributes are evaluated in the order they appear in the DOM. Elements are evaluated by walking the DOM in a depth-first manner, and attributes are processed in the order they appear in the element. This means that if you use a signal in a [Datastar expression](/guide/datastar_expressions), it must be defined _before_ it is used.

```html
<!-- This works: -->
<div data-signals-foo="1" data-text="$foo"></div>

<!-- This works: -->
<div data-signals-foo="1"></div>
<div data-text="$foo"></div>

<!-- This works: -->
<div data-signals-foo="1">
  <div data-text="$foo"></div>
</div>

<!-- This does NOT work: -->
<div data-text="$foo" data-signals-foo="1"></div>

<!-- This does NOT work: -->
<div data-text="$foo"></div>
<div data-signals-foo="1"></div>
```

## Core Plugins

[Source Code](https://github.com/starfederation/datastar/blob/main/library/src/plugins/official/core/attributes)

The core plugins are included in every bundle, and contain the core functionality in Datastar.

### `data-signals`

Merges one or more signals into the existing signals. Values defined later in the DOM tree override those defined earlier.

```html
<div data-signals-foo="1"></div>
```

Signals can be namespaced using dot-notation.

```html
<div data-signals-foo.bar="1"></div>
```

Note when working with namespaced signals that only the leaf nodes are actually signals. So in the example above, only `bar` is a signal, meaning that while using `$foo.bar` in an expression is possible, using `$foo` (the namespace) is not.

The `data-signals` attribute can also be used to merge multiple signals using a set of key-value pairs, where the keys represent signal names and the values represent expressions.

```html
<div data-signals="{foo: {bar: 1, baz: 2}}"></div>
```

The value above is written in JavaScript object notation, but JSON, which is a subset and which most templating languages have built-in support for, is also allowed.

Note that `data-*` attributes are case-insensitive. If you want to use uppercase characters in signal names, you'll need to kebabize them or use object syntax. So the signal name `mySignal` must be written as `data-signals-my-signal` or `data-signals="{mySignal: 1}"`.

You can further modify the casing of keys in `data-*` attributes using the `__case` modifier, followed by `.kebab`, `.snake`, or `.pascal`.

Signals beginning with an underscore are considered _local signals_ and are not included in requests to the backend by default. You can include them by setting the [`includeLocal`](/reference/action_plugins#options) option to `true`.

Signal names cannot begin or contain double underscores (`__`), due to the use of `__` as a modifer delimiter.

#### Modifiers

Modifiers allow you to modify behavior when merging signals.

- `__ifmissing` - Only merges signals if their keys do not already exist. This is useful for setting defaults without overwriting existing values.

```html
<div data-signals-foo__ifmissing="1"></div>
```

### `data-computed`

Creates a signal that is computed based on an expression. The computed signal is read-only, and its value is automatically updated when any signals in the expression are updated.

```html
<div data-computed-foo="$bar + $baz"></div>
```

Computed signals are useful for memoizing expressions containing other signals. Their values can be used in other expressions.

```html
<div data-computed-foo="$bar + $baz"></div>
<div data-text="$foo"></div>
```

### `data-ref`

Creates a new signal that is a reference to the element on which the data attribute is placed.

```html
<div data-ref-foo></div>
```

The signal name can be specified in the key (as above), or in the value (as below). This can be useful depending on the templating language you are using.

```html
<div data-ref="foo"></div>
```

The signal value can then be used to reference the element.

```html
`foo` holds a <span data-text="$foo.tagName"></span> element.
```

## DOM Plugins

[Source Code](https://github.com/starfederation/datastar/blob/main/library/src/plugins/official/dom/attributes)

Allows the usage of signals and expressions to affect the DOM.

### `data-attr`

Sets the value of any HTML attribute to an expression, and keeps it in sync.

```html
<div data-attr-title="$foo"></div>
```

The `data-attr` attribute can also be used to set the values of multiple attributes on an element using a set of key-value pairs, where the keys represent attribute names and the values represent expressions.

```html
<div data-attr="{title: $foo, disabled: $bar}"></div>
```

### `data-bind`

Creates a signal (if one doesn't already exist) and sets up two-way data binding between it and an element's value. This means that the value of the element is updated when the signal changes, and the signal is updated when the value of the element changes.

The `data-bind` attribute be placed on any HTML element on which data can be input or choices selected from (`input`, `select`,`textarea` elements, and web components). Event listeners are added for `change`, `input` and `keydown` events.

```html
<input data-bind-foo />
```

The signal name can be specified in the key (as above), or in the value (as below). This can be useful depending on the templating language you are using.

```html
<input data-bind="foo" />
```

The initial value of the signal is set to the value of the element, unless a signal has already been defined. So in the example below, `$foo` is set to `bar`.

```html
<input data-bind-foo value="bar" />
```

Whereas in the example below, `$foo` inherits the value `baz` of the predefined signal.

```html
<div data-signals-foo="baz">
  <input data-bind-foo value="bar" />
</div>
```

Multiple input values can be assigned to a single signal by predefining the signal as an array. So in the example below, `$foo` is set to `['bar', 'baz']` when both checkboxes are checked.

```html
<div data-signals-foo="[]">
  <input data-bind-foo type="checkbox" value="bar" />
  <input data-bind-foo type="checkbox" value="baz" />
</div>
```

### `data-class`

Adds or removes a class to or from an element based on an expression.

```html
<div data-class-hidden="$foo"></div>
```

If the expression evaluates to `true`, the `hidden` class is added to the element; otherwise, it is removed.

The `data-class` attribute can also be used to add or remove multiple classes from an element using a set of key-value pairs, where the keys represent class names and the values represent expressions.

```html
<div data-class="{hidden: $foo, 'font-bold': $bar}"></div>
```

### `data-on`

Attaches an event listener to an element, executing an expression whenever the event is triggered.

```html
<button data-on-click="$foo = ''">Reset</button>
```

An `evt` variable that represents the event object is available in the expression.

```html
<div data-on-myevent="$foo = evt.detail"></div>
```

The `data-on` attribute works with [built-in events](https://developer.mozilla.org/en-US/docs/Web/Events) and [custom events](https://developer.mozilla.org/en-US/docs/Web/Events/Creating_and_triggering_events). Note that the `data-on-submit` event listener prevents the default submission behavior of forms.

#### Special Events

Datastar provides a few special events of its own:

1. `data-on-load` is triggered when an element is loaded into the DOM.
2. `data-on-interval` is triggered at a regular interval. The interval duration defaults to 1 second and can be modified using the `__duration` modifier.
3. `data-on-raf` is triggered on every [`requestAnimationFrame`](https://developer.mozilla.org/en-US/docs/Web/API/Window/requestAnimationFrame) event.
4. `data-on-resize` is triggered whenever the element's dimensions change.
5. `data-on-signals-change` is triggered when any signals change. A key can be provided to only trigger the event when the signal with that key changes (`data-on-signals-change-foo`).

Note that the `evt` variable is _not_ available in the expression when using special events.

#### Modifiers

Modifiers allow you to modify behavior when events are triggered. Some modifiers have tags to further modify the behavior.

- `__once` \* - Only trigger the event listener once.
- `__passive` \* - Do not call `preventDefault` on the event listener.
- `__capture` \* - Use a capture event listener.
- `__delay` - Delay the event listener.
  - `.500ms` - Delay for 500 milliseconds.
  - `.1s` - Delay for 1 second.
- `__debounce` - Debounce the event listener.
  - `.500ms` - Debounce for 500 milliseconds.
  - `.1s` - Debounce for 1 second.
  - `.leading` - Debounce with leading edge.
  - `.notrail` - Debounce without trailing edge.
- `__throttle` - Throttle the event listener.
  - `.500ms` - Throttle for 500 milliseconds.
  - `.1s` - Throttle for 1 second.
  - `.noleading` - Throttle without leading edge.
  - `.trail` - Throttle with trailing edge.
- `__duration` - Sets the interval duration for `data-on-interval`.
  - `.500ms` - Interval duration of 500 milliseconds.
  - `.1s` - Interval duration of 1 second.
  - `.leading` - Execute the first interval immediately.
- `__viewtransition` - Wraps the expression in `document.startViewTransition()` when the View Transition API is available.
- `__window` - Attaches the event listener to the `window` element.
- `__outside` - Triggers when the event is outside the element.
- `__prevent` - Calls `preventDefault` on the event listener.
- `__stop` - Calls `stopPropagation` on the event listener.

\* Only works on built-in events.

```html
<div data-on-click__window__debounce.500ms.leading="$foo = ''"></div>
```

### `data-persist`

Persists signals in Local Storage. This is useful for storing values between page loads.

```html
<div data-persist></div>
```

If one or more space-separated values are provided as a string, only those signals are persisted.

```html
<div data-persist="foo bar"></div>
```

If a key is provided, it will be used as the key when saving in storage, otherwise `datastar` will be used.

```html
<div data-persist-mykey="foo bar"></div>
```

#### Modifiers

Modifiers allow you to modify the storage target.

- `__session` - Persists signals in Session Storage.

```html
<div data-persist__session></div>
```

### `data-replace-url`

Replaces the URL in the browser without reloading the page. The value can be a relative or absolute URL, and is an evaluated expression.

```html
<div data-replace-url="`/page${page}`"></div>
```

### `data-text`

Binds the text content of an element to an expression.

```html
<div data-text="$foo"></div>
```

## Browser Plugins

[Source Code](https://github.com/starfederation/datastar/tree/main/library/src/plugins/official/browser/attributes)

Focused on showing and hiding elements based on signals. Most of the time you want to send updates from the server but is useful for things like modals, dropdowns, and other UI elements.

### `data-custom-validity`

Allows you to add custom validity to an element using an expression. The expression must evaluate to a string that will be set as the custom validity message. If the string is empty, the input is considered valid. If the string is non-empty, the input is considered invalid and the string is used as the reported message.

```html
<form>
  <input data-bind-foo name="foo" />
  <input data-bind-bar name="bar"
    data-custom-validity="$foo === $bar ? '' : 'Field values must be the same.'" 
  />
  <button>Submit form</button>
</form>
```

### `data-intersects`

Runs an expression when the element intersects with the viewport.

```html
<div data-intersects="$intersected = true"></div>
```

#### Modifiers

Modifiers allow you to modify the element intersection behavior.

- `__once` - Only triggers the event once.
- `__half` - Triggers when half of the element is visible.
- `__full` - Triggers when the full element is visible.

```html
<div data-intersects__once="$intersected = true"></div>
```

### `data-scroll-into-view`

Scrolls the element into view. Useful when updating the DOM from the backend, and you want to scroll to the new content.

```html
<div data-scroll-into-view></div>
```

#### Modifiers

Modifiers allow you to modify scrolling behavior.

- `__smooth` - Scrolling is animate smoothly.
- `__instant` - Scrolling is instant.
- `__auto` - Scrolling is determined by the computed `scroll-behavior` CSS property.
- `__hstart` - Scrolls to the left of the element.
- `__hcenter` - Scrolls to the horizontal center of the element.
- `__hend` - Scrolls to the right of the element.
- `__hnearest` - Scrolls to the nearest horizontal edge of the element.
- `__vstart` - Scrolls to the top of the element.
- `__vcenter` - Scrolls to the vertical center of the element.
- `__vend` - Scrolls to the bottom of the element.
- `__vnearest` - Scrolls to the nearest vertical edge of the element.
- `__focus` - Focuses the element after scrolling.

```html
<div data-scroll-into-view__smooth></div>
```

### `data-show`

Show or hides an element based on whether an expression evaluates to `true` or `false`. For anything with custom requirements, use [`data-class`](#data-class) instead.

```html
<div data-show="$foo"></div>
```

To prevent flickering of the element before Datastar has processed the DOM, you can add a `display: none` style to the element to hide it initially.

```html
<div data-show="$foo" style="display: none"></div>
```

### `data-view-transition`

Sets the `view-transition-name` style attribute explicitly.

```html
<div data-view-transition="$foo"></div>
```

Page level transitions are automatically handled by an injected meta tag. Inter-page elements are automatically transitioned if the [View Transition API](https://developer.mozilla.org/en-US/docs/Web/API/View_Transitions_API) is available in the browser and `useViewTransitions` is `true`.

## Backend Plugins

[Source Code](https://github.com/starfederation/datastar/blob/main/library/src/plugins/official/backend/attributes)

Adds integrations with [backend plugin actions](/reference/action_plugins#backend-plugins).

### `data-indicator`

Creates a signal and sets its value to `true` while an SSE request request is in flight, otherwise `false`. The signal can be used to show a loading indicator.

```html
<button data-on-click="@get('/endpoint')" data-indicator-fetching></button>
```

This can be useful for show a loading spinner, disabling a button, etc.

```html
<button
  data-on-click="@get('/endpoint')"
  data-indicator-fetching
  data-attr-disabled="$fetching"
></button>
<div data-show="$fetching">Loading...</div>
```

The signal name can be specified in the key (as above), or in the value (as below). This can be useful depending on the templating language you are using.

```html
<button data-indicator="fetching"></button>
```

## Ignoring Elements

### `data-star-ignore`

Datastar walks the entire DOM and applies plugins to each element it encounters. It's possible to tell Datastar to ignore an element and its descendants by placing a `data-star-ignore` attribute on it. This can be useful for preventing naming conflicts with third-party libraries, or when you are unable to [escape user input](/reference/security#escape-user-input).

```html
<div data-star-ignore data-show-thirdpartylib>
  <div data-show-thirdpartylib>
    These element will not be processed by Datastar.
  </div>
</div>
```

#### Modifiers

- `__self` - Only ignore the element itself, not its descendants.


## Aliasing Data Attributes

It is possible to alias `data-*` attributes to a custom alias (`data-foo-*`, for example) using the [bundler](/bundler). A custom alias should _only_ be used if you have a conflict with a legacy library and [`data-star-ignore`](#data-star-ignore) cannot be used.

We maintain a `data-ds-*` aliased version that can be included as follows.

```html
<script type="module" src="https://cdn.jsdelivr.net/gh/starfederation/datastar@v1.0.0-beta.9/bundles/datastar-aliased.js"></script>
```