# Switch

Turn a setting on or off.

Use a switch for a setting that takes effect as soon as it changes, like Airplane mode or a notification preference. Keep its label the same in both states: the label says what the setting is, and the switch shows whether it's on.

```svelte title="basic.svelte"
<script lang="ts">
  import { Switch, SwitchThumb } from "@ui-sdk/svelte/switch"
</script>

<label data-slot="label">
  Airplane mode
  <Switch>
    <SwitchThumb />
  </Switch>
</label>
```

Click Airplane mode, or focus the switch and press Space, to turn it on and off.

## Installation

```bash
npm install @ui-sdk/svelte @ui-sdk/core
```

## Composition

Use the following composition to build a `Switch`:

```text
Switch
└── SwitchThumb
```

Wrap the switch and its text in a label. The thumb stays mounted and gets the switch's state, so CSS can slide it from one end to the other.

## Usage

Import the parts and put them together:

```svelte
<script lang="ts">
  import { Switch, SwitchThumb } from "@ui-sdk/svelte/switch"
</script>

<label>
  Airplane mode
  <Switch>
    <SwitchThumb />
  </Switch>
</label>
```

## Description

Use `FieldLabel` and `FieldDescription` when a setting needs more explanation. The field connects both to the switch.

```svelte title="description.svelte"
<script lang="ts">
  import { Field, FieldDescription, FieldLabel } from "@ui-sdk/svelte/field"
  import { Switch, SwitchThumb } from "@ui-sdk/svelte/switch"
</script>

<Field>
  <div data-slot="stack">
    <FieldLabel>Weekly digest</FieldLabel>
    <FieldDescription>A summary of your week, every Monday.</FieldDescription>
  </div>
  <Switch>
    <SwitchThumb />
  </Switch>
</Field>
```

Click Weekly digest. The field connects the label and description to the switch.

## Choice Card

A setting can fill a whole card. Here the label's `::after` covers the card, so a click anywhere on it toggles the switch, and `:has([data-checked])` marks the cards that are on.

```svelte title="choice-card.svelte"
<script lang="ts">
  import { Field, FieldDescription, FieldLabel } from "@ui-sdk/svelte/field"
  import { Switch, SwitchThumb } from "@ui-sdk/svelte/switch"

  const settings = [
    {
      label: "Public profile",
      description: "Anyone can see your name and photo.",
      defaultChecked: false,
    },
    {
      label: "Show activity status",
      description: "Teammates can see when you're online.",
      defaultChecked: true,
    },
  ]
</script>

<div data-slot="stack">
  {#each settings as setting (setting.label)}
    <Field data-variant="card">
      <div data-slot="stack">
        <FieldLabel>{setting.label}</FieldLabel>
        <FieldDescription>{setting.description}</FieldDescription>
      </div>
      <Switch defaultChecked={setting.defaultChecked}>
        <SwitchThumb />
      </Switch>
    </Field>
  {/each}
</div>
```

Click anywhere on the Public profile card to turn it on. Each card's switch keeps its own value.

## Disabled

`disabled` prevents focus and changes, and leaves the switch out of form submissions. A disabled `fieldset` does the same.

```svelte title="disabled.svelte"
<script lang="ts">
  import { Switch, SwitchThumb } from "@ui-sdk/svelte/switch"
</script>

<div data-slot="stack">
  <label data-slot="label">
    Product updates
    <Switch>
      <SwitchThumb />
    </Switch>
  </label>
  <label data-slot="label">
    Security alerts
    <Switch defaultChecked disabled>
      <SwitchThumb />
    </Switch>
  </label>
</div>
```

Security alerts are always on, so that switch is disabled. Clicking it leaves it on, and Tab skips it.

## Read-Only

`readOnly` keeps the switch focusable and included in form submissions, while preventing the user from changing it. Use it for a setting managed somewhere else.

```svelte title="read-only.svelte"
<script lang="ts">
  import { Field, FieldDescription, FieldLabel } from "@ui-sdk/svelte/field"
  import { Switch, SwitchThumb } from "@ui-sdk/svelte/switch"
</script>

<Field>
  <div data-slot="stack">
    <FieldLabel>Encrypt backups</FieldLabel>
    <FieldDescription>Your organization manages this setting.</FieldDescription>
  </div>
  <Switch name="encrypt" defaultChecked readOnly>
    <SwitchThumb />
  </Switch>
</Field>
```

Tab to Encrypt backups. Space and clicks leave it on.

## Controlled

Bind `bind:checked`, or pass `checked` and handle `onCheckedChange`, to own the value. A [controller](#controller) from `createSwitchController()` changes it from anywhere, like the Toggle button below. See [State and Events](/docs/svelte/state-and-events) for the shared change callback.

```svelte title="controlled.svelte"
<script lang="ts">
  import { createSwitchController } from "@ui-sdk/core/switch"
  import { Switch, SwitchThumb } from "@ui-sdk/svelte/switch"
  import { useLog } from "./log"

  const log = useLog()
  const wifi = createSwitchController()
  let checked = $state(true)
</script>

<div data-slot="stack">
  <div data-slot="controls">
    <button type="button" onclick={() => wifi.toggle()}>Toggle</button>
  </div>
  <label data-slot="label">
    Wi-Fi
    <Switch
      bind:checked
      onCheckedChange={(next, details) =>
        log(`${next ? "On" : "Off"} (${details.reason})`)}
      controller={wifi}
    >
      <SwitchThumb />
    </Switch>
  </label>
</div>
```

Turn Wi-Fi off, or press Toggle. The console logs each change and its reason.

## Form

Set `name` to include the switch in a native form. A switch that's on submits its `value`, which defaults to `"on"`; a switch that's off submits nothing. Reset returns it to `defaultChecked`. Set `form` to a form's id to join it from elsewhere on the page.

```svelte title="form.svelte"
<script lang="ts">
  import { FieldLegend, FieldSet } from "@ui-sdk/svelte/field"
  import { Switch, SwitchThumb } from "@ui-sdk/svelte/switch"
  import { useLog } from "./log"

  const log = useLog()
</script>

<form
  data-slot="stack"
  onsubmit={(event) => {
    event.preventDefault()
    const entries = [...new FormData(event.currentTarget)].map(
      ([name, value]) => `${name}=${value}`
    )
    log(entries.length ? `Submitted ${entries.join("&")}` : "Submitted nothing")
  }}
>
  <FieldSet>
    <FieldLegend>Email notifications</FieldLegend>
    <label data-slot="label">
      Product updates
      <Switch name="updates">
        <SwitchThumb />
      </Switch>
    </label>
    <label data-slot="label">
      Security alerts
      <Switch name="security" defaultChecked>
        <SwitchThumb />
      </Switch>
    </label>
    <label data-slot="label">
      Weekly digest
      <Switch name="digest" value="weekly" defaultChecked>
        <SwitchThumb />
      </Switch>
    </label>
  </FieldSet>
  <div data-slot="row">
    <button type="submit" data-variant="solid">Save</button>
    <button type="reset">Reset</button>
  </div>
</form>
```

Turn on Product updates and press Save. The console logs the submitted values, and Reset restores the defaults.

## Validation

`required` blocks submission until the switch is on. In a [Field](/docs/svelte/components/field), `FieldError` shows the message under the switch and connects it to the switch. [Forms and Validation](/docs/svelte/forms) covers the shared field behavior.

```svelte title="invalid.svelte"
<script lang="ts">
  import {
    Field,
    FieldDescription,
    FieldError,
    FieldLabel,
  } from "@ui-sdk/svelte/field"
  import { Switch, SwitchThumb } from "@ui-sdk/svelte/switch"
  import { useLog } from "./log"

  const log = useLog()
</script>

<form
  data-slot="stack"
  onsubmit={(event) => {
    event.preventDefault()
    log(
      `Submitted two-factor=${new FormData(event.currentTarget).get("two-factor")}`
    )
  }}
>
  <Field>
    <div data-slot="row">
      <div data-slot="stack">
        <FieldLabel>Two-factor authentication</FieldLabel>
        <FieldDescription
          >Required for admins of this workspace.</FieldDescription
        >
      </div>
      <Switch name="two-factor" required>
        <SwitchThumb />
      </Switch>
    </div>
    <FieldError match="valueMissing">Turn this on to continue.</FieldError>
  </Field>
  <div data-slot="row">
    <button type="submit" data-variant="solid">Continue</button>
  </div>
</form>
```

Press Continue with the switch off to see the error. Turn on Two-factor authentication and continue; the console logs the submitted value.

## Saving on Toggle

To save a change before it shows, call `details.cancel()` in `onCheckedChange` to keep the current value while the save runs. When it finishes, call `controller.setChecked(next, { reason: "imperative" })`, and let that reason through in the callback so the saved value commits.

The example ignores presses while a save is running, and shows a spinner in the thumb. `aria-busy` tells assistive technology the switch is busy; the callback is what blocks another change.

```svelte title="async.svelte"
<script lang="ts">
  import { LoaderCircleIcon } from "@lucide/svelte"
  import { createSwitchController } from "@ui-sdk/core/switch"
  import { Switch, SwitchThumb } from "@ui-sdk/svelte/switch"
  import { useLog } from "./log"

  const save = (_checked: boolean) =>
    new Promise((resolve) => setTimeout(resolve, 800))

  const log = useLog()
  const backups = createSwitchController()
  let saving = $state(false)
</script>

<label data-slot="label">
  Automatic backups
  <Switch
    controller={backups}
    aria-busy={saving}
    onCheckedChange={(next, details) => {
      if (details.reason === "imperative") {
        return
      }
      details.cancel()
      if (saving) {
        return
      }
      saving = true
      log("Saving…")
      save(next).then(() => {
        backups.setChecked(next, { reason: "imperative" })
        saving = false
        log(`Saved: ${next ? "on" : "off"}`)
      })
    }}
  >
    <SwitchThumb>
      {#if saving}<LoaderCircleIcon
          data-slot="spinner"
          aria-hidden="true"
        />{/if}
    </SwitchThumb>
  </Switch>
</label>
```

Turn on Automatic backups. The thumb shows a spinner while it saves, then the switch turns on.

## Custom Rendering

The switch and thumb take a `render` snippet, which receives the props to spread and the part's state. Keep the part's props on the element so its role, label and keyboard behavior stay connected. See [Parts and Composition](/docs/svelte/parts-and-composition) for the rendering contract.

```svelte title="render.svelte"
<script lang="ts">
  import { MoonIcon, SunIcon } from "@lucide/svelte"
  import { Switch, SwitchThumb } from "@ui-sdk/svelte/switch"
</script>

<label data-slot="label">
  Dark mode
  <Switch>
    <SwitchThumb>
      {#snippet render({ props, state })}
        <span {...props}>
          {#if state.checked}<MoonIcon aria-hidden="true" />{:else}<SunIcon
              aria-hidden="true"
            />{/if}
        </span>
      {/snippet}
    </SwitchThumb>
  </Switch>
</label>
```

Turn on Dark mode. The thumb renders a sun or a moon from its state.

## Styling

Style the switch and its thumb with `data-checked` and `data-unchecked`, and use `data-disabled`, `data-readonly` and `data-required` for the rest. In an invalid field, the switch also gets `data-invalid` and `aria-invalid`; the thumb doesn't. Slide the thumb with `translate` on `data-checked`, and reverse it under `dir="rtl"`.

Each part has a `data-slot`, such as `switch`, and its state attributes are listed under [Data Attributes](#data-attributes).

## Data Attributes

### Switch
`[data-slot="switch"]`

- `data-checked`: Present while on.
- `data-disabled`: Present when disabled, by its own prop or a `<fieldset>`.
- `data-readonly`: Present when read-only.
- `data-required`: Present when required.
- `data-unchecked`: Present while off.
- `data-invalid`: Present while a `Field` around it shows as invalid. The thumb doesn't get it.

### SwitchThumb
`[data-slot="switch-thumb"]`

- `data-checked`: Present while on.
- `data-disabled`: Present when disabled, by its own prop or a `<fieldset>`.
- `data-readonly`: Present when read-only.
- `data-required`: Present when required.
- `data-unchecked`: Present while off.

## API Reference

Props and callbacks are the same as below, as Svelte props. `checked` can be bound with `bind:checked`.

### Switch

- `checked?` (`boolean`): Whether it's checked. Pass it to control the checkbox.
- `defaultChecked?` (`boolean`): Whether it's checked at first, when uncontrolled. A form reset returns to it.
- `onCheckedChange?` (`((checked: boolean, details: ChangeDetails<CheckboxCheckedChangeReason>) => void)`): Runs when a user or a form reset wants to change it. `details.reason` says why, and `details.cancel()` stops it.
- `disabled?` (`boolean`): Disables it like a native control: it can't be focused or changed, and isn't submitted.
- `readOnly?` (`boolean`): Keeps it focusable and submitted, but stops changes.
- `required?` (`boolean`): Blocks form submission until it's checked, with the browser's own message.
- `name?` (`string`): The name it submits under. Without one, it submits nothing.
- `value?` (`string`): What it submits while checked.
- `form?` (`string`): The id of a form to belong to, when it isn't inside that form.
- `controller?` (`{ toggle: (details?: Details) => boolean; getChecked: () => boolean; setChecked: (checked: boolean, details?: Details) => boolean; focus: () => void; }`): Toggles it from anywhere. Create one with `createSwitchController()`.
- `debug?` (`boolean`): Logs every state change with its reason to the console, in development builds.

### State

What `render` and `children` snippets receive. The thumb's state, `SwitchThumbState`, is the same.

- `checked` (`boolean`): Whether it's checked. False while indeterminate.
- `disabled` (`boolean`): Whether it's disabled, by itself or by a `<fieldset>`.
- `readOnly` (`boolean`): Whether it's read-only: it stays focusable and submitted, but presses don't change it.
- `required` (`boolean`): Whether a form won't submit until it's checked. Indeterminate doesn't count.

### Reasons

`details.reason` says why the switch turned on or off.

- `press` (`MouseEvent | KeyboardEvent`): A single-element control was pressed: a checkbox, a switch or a toggle.
- `reset` (`none`): A form reset.
- `restore` (`none`): The browser restored state, like after going back, or a saved value was read back from storage.
- `imperative` (`none`): A controller or `api` call, or a property set on a web component.

### Controller

Create one with `createSwitchController()` and pass it as `controller`.

Every write method takes an optional `{ reason, event }`, runs the change callback like a press, and returns `false` when the change is canceled.

- `toggle(details?)` (`(details?: CheckboxChangeOptions | undefined) => boolean`): Turns it on, or off.
- `getChecked()` (`() => boolean`): Whether it's on.
- `setChecked(checked, details?)` (`(checked: boolean, details?: CheckboxChangeOptions | undefined) => boolean`): Turns it on or off, as a user would.
- `focus()` (`() => void`): Moves focus to it.

### Events

Callbacks get the new state first, then the details. A bound `checked` only updates when `onCheckedChange` wasn't canceled.

| Prop              | On       | Receives                                                        |
| ----------------- | -------- | --------------------------------------------------------------- |
| `bind:checked`    | `Switch` | Whether the switch is on                                        |
| `onCheckedChange` | `Switch` | `(checked, details)`: the new state, and the [reason](#reasons) |

## Accessibility

The switch follows the [WAI-ARIA switch pattern](https://www.w3.org/WAI/ARIA/apg/patterns/switch/). `Switch` renders a button with the `switch` role, and a hidden native checkbox that takes part in forms.

### Keyboard

| Key   | Does                                   |
| ----- | -------------------------------------- |
| Tab   | Moves to the next switch               |
| Space | Turns it on or off                     |
| Enter | Submits its form, and never toggles it |

### Tested Behavior

Every line below is an automated test.

- Starts in its default state.
- Clicking toggles it.
- Clicking its label toggles it.
- Space toggles it.
- Enter submits the form and never toggles it.
- The thumb follows the state.

**The first input on a page**

- The first Space reports keyboard.

**On a touch screen**

- The first tap reports touch.

Runs in React, Vue and web components.

In forms:

- The form submits every checked control's name and value.
- Unchecked controls submit nothing.
- A checkbox outside the form joins it with the form attribute.
- Required controls block submission.
- A form reset returns every control to its default.
- A reset returns to the default value, not to nothing.
- A disabled fieldset disables the controls inside it.
- A read-only control still submits.
- A user's change fires native input and change events on the form.

Runs in React, Vue and web components.

Beyond the pattern's baseline:

**Enhancements**

- The first invalid control takes focus in document order.
- A later required field without an error part doesn't take focus from the first.

**Form-associated custom elements**

- Take part in the form with no hidden input. Web components only.
- Report validity like native controls. Web components only.
- Have live checked and value properties. Web components only.
- Are named by their labels. Web components only.

Runs in React, Vue and web components.

Screen readers hear the same thing in each tested framework, recorded by a virtual screen reader:

- Announcements while checking boxes with a keyboard.
- Announcements while choosing a radio with a keyboard.

Runs in React, Vue and web components.