<s>[INST] The following document is part of the VueJS documentation, in the file vuejs_docs/src/guide/components/attrs.md. Please understand it and learn it to generate quality code in the future. [/INST] ---
outline: deep
---

# Fallthrough Attributes {#fallthrough-attributes}

> This page assumes you've already read the [Components Basics](/guide/essentials/component-basics). Read that first if you are new to components.

## Attribute Inheritance {#attribute-inheritance}

A "fallthrough attribute" is an attribute or `v-on` event listener that is passed to a component, but is not explicitly declared in the receiving component's [props](./props) or [emits](./events#declaring-emitted-events). Common examples of this include `class`, `style`, and `id` attributes.

When a component renders a single root element, fallthrough attributes will be automatically added to the root element's attributes. For example, given a `<MyButton>` component with the following template:

```vue-html
<!-- template of <MyButton> -->
<button>click me</button>
```

And a parent using this component with:

```vue-html
<MyButton class="large" />
```

The final rendered DOM would be:

```html
<button class="large">click me</button>
```

Here, `<MyButton>` did not declare `class` as an accepted prop. Therefore, `class` is treated as a fallthrough attribute and automatically added to `<MyButton>`'s root element.

### `class` and `style` Merging {#class-and-style-merging}

If the child component's root element already has existing `class` or `style` attributes, it will be merged with the `class` and `style` values that are inherited from the parent. Suppose we change the template of `<MyButton>` in the previous example to:

```vue-html
<!-- template of <MyButton> -->
<button class="btn">click me</button>
```

Then the final rendered DOM would now become:

```html
<button class="btn large">click me</button>
```

### `v-on` Listener Inheritance {#v-on-listener-inheritance}

The same rule applies to `v-on` event listeners:

```vue-html
<MyButton @click="onClick" />
```

The `click` listener will be added to the root element of `<MyButton>`, i.e. the native `<button>` element. When the native `<button>` is clicked, it will trigger the `onClick` method of the parent component. If the native `<button>` already has a `click` listener bound with `v-on`, then both listeners will trigger.

### Nested Component Inheritance {#nested-component-inheritance}

If a component renders another component as its root node, for example, we refactored `<MyButton>` to render a `<BaseButton>` as its root:

```vue-html
<!-- template of <MyButton/> that simply renders another component -->
<BaseButton />
```

Then the fallthrough attributes received by `<MyButton>` will be automatically forwarded to `<BaseButton>`.

Note that:

1. Forwarded attributes do not include any attributes that are declared as props, or `v-on` listeners of declared events by `<MyButton>` - in other words, the declared props and listeners have been "consumed" by `<MyButton>`.

2. Forwarded attributes may be accepted as props by `<BaseButton>`, if declared by it.

## Disabling Attribute Inheritance {#disabling-attribute-inheritance}

If you do **not** want a component to automatically inherit attributes, you can set `inheritAttrs: false` in the component's options.

<div class="composition-api">

 Since 3.3 you can also use [`defineOptions`](/api/sfc-script-setup#defineoptions) directly in `<script setup>`:

```vue
<script setup>
defineOptions({
  inheritAttrs: false
})
// ...setup logic
</script>
```

</div>

The common scenario for disabling attribute inheritance is when attributes need to be applied to other elements besides the root node. By setting the `inheritAttrs` option to `false`, you can take full control over where the fallthrough attributes should be applied.

These fallthrough attributes can be accessed directly in template expressions as `$attrs`:

```vue-html
<span>Fallthrough attributes: {{ $attrs }}</span>
```

The `$attrs` object includes all attributes that are not declared by the component's `props` or `emits` options (e.g., `class`, `style`, `v-on` listeners, etc.).

Some notes:

- Unlike props, fallthrough attributes preserve their original casing in JavaScript, so an attribute like `foo-bar` needs to be accessed as `$attrs['foo-bar']`.

- A `v-on` event listener like `@click` will be exposed on the object as a function under `$attrs.onClick`.

Using our `<MyButton>` component example from the [previous section](#attribute-inheritance) - sometimes we may need to wrap the actual `<button>` element with an extra `<div>` for styling purposes:

```vue-html
<div class="btn-wrapper">
  <button class="btn">click me</button>
</div>
```

We want all fallthrough attributes like `class` and `v-on` listeners to be applied to the inner `<button>`, not the outer `<div>`. We can achieve this with `inheritAttrs: false` and `v-bind="$attrs"`:

```vue-html{2}
<div class="btn-wrapper">
  <button class="btn" v-bind="$attrs">click me</button>
</div>
```

Remember that [`v-bind` without an argument](/guide/essentials/template-syntax#dynamically-binding-multiple-attributes) binds all the properties of an object as attributes of the target element.

## Attribute Inheritance on Multiple Root Nodes {#attribute-inheritance-on-multiple-root-nodes}

Unlike components with a single root node, components with multiple root nodes do not have an automatic attribute fallthrough behavior. If `$attrs` are not bound explicitly, a runtime warning will be issued.

```vue-html
<CustomLayout id="custom-layout" @click="changeValue" />
```

If `<CustomLayout>` has the following multi-root template, there will be a warning because Vue cannot be sure where to apply the fallthrough attributes:

```vue-html
<header>...</header>
<main>...</main>
<footer>...</footer>
```

The warning will be suppressed if `$attrs` is explicitly bound:

```vue-html{2}
<header>...</header>
<main v-bind="$attrs">...</main>
<footer>...</footer>
```

## Accessing Fallthrough Attributes in JavaScript {#accessing-fallthrough-attributes-in-javascript}

<div class="composition-api">

If needed, you can access a component's fallthrough attributes in `<script setup>` using the `useAttrs()` API:

```vue
<script setup>
import { useAttrs } from 'vue'

const attrs = useAttrs()
</script>
```

If not using `<script setup>`, `attrs` will be exposed as a property of the `setup()` context:

```js
export default {
  setup(props, ctx) {
    // fallthrough attributes are exposed as ctx.attrs
    console.log(ctx.attrs)
  }
}
```

Note that although the `attrs` object here always reflects the latest fallthrough attributes, it isn't reactive (for performance reasons). You cannot use watchers to observe its changes. If you need reactivity, use a prop. Alternatively, you can use `onUpdated()` to perform side effects with the latest `attrs` on each update.

</div>

<div class="options-api">

If needed, you can access a component's fallthrough attributes via the `$attrs` instance property:

```js
export default {
  created() {
    console.log(this.$attrs)
  }
}
```

</div>
 </s>