# `<svelte:boundary>`

```svelte
<svelte:boundary onerror={handler}>...</svelte:boundary>
```

> [!NOTE]
> This feature was added in 5.3.0

Boundaries allow you to 'wall off' parts of your app, so that you can:

- provide UI that should be shown when [`await`](await-expressions) expressions are first resolving
- handle errors that occur during rendering or while running effects, and provide UI that should be rendered when an error happens

If a boundary handles an error (with a `failed` snippet or `onerror` handler, or both) its existing content will be removed.

> [!NOTE] Errors occurring outside the rendering process (for example, in event handlers or after a `setTimeout` or async work) are _not_ caught by error boundaries.

## Properties

For the boundary to do anything, one or more of the following must be provided.

### `pending`

This snippet will be shown when the boundary is first created, and will remain visible until all the [`await`](await-expressions) expressions inside the boundary have resolved ([demo](/playground/untitled#H4sIAAAAAAAAE21QQW6DQAz8ytY9BKQVpFdKkPqDHnorPWzAaSwt3tWugUaIv1eE0KpKD5as8YxnNBOw6RAKKOOAVrA4up5bEy6VGknOyiO3xJ8qMnmPAhpOZDFC8T6BXPyiXADQ258X77P1FWg4moj_4Y1jQZZ49W0CealqruXUcyPkWLVozQXbZDC2R606spYiNo7bqA7qab_fp2paFLUElD6wYhzVa3AdRUySgNHZAVN1qDZaLRHljTp0vSTJ9XJjrSbpX5f0eZXN6zLXXOa_QfmurIVU-moyoyH5ib87o7XuYZfOZe6vnGWmx1uZW7lJOq9upa-sMwuUZdkmmfIbfQ1xZwwaBL8ECgk9zh8axJAdiVsoTsZGnL8Bg4tX_OMBAAA=)):

```svelte
<svelte:boundary>
	<p>{await delayed('hello!')}</p>

	{#snippet pending()}
		<p>loading...</p>
	{/snippet}
</svelte:boundary>
```

The `pending` snippet will _not_ be shown for subsequent async updates — for these, you can use [`$effect.pending()`]($effect#$effect.pending).

> [!NOTE] In the [playground](/playground), your app is rendered inside a boundary with an empty pending snippet, so that you can use `await` without having to create one.


### `failed`

If a `failed` snippet is provided, it will be rendered when an error is thrown inside the boundary, with the `error` and a `reset` function that recreates the contents ([demo](/playground/hello-world#H4sIAAAAAAAAE3VRy26DMBD8lS2tFCIh6JkAUlWp39Cq9EBg06CAbdlLArL87zWGKk8ORnhmd3ZnrD1WtOjFXqKO2BDGW96xqpBD5gXerm5QefG39mgQY9EIWHxueRMinLosti0UPsJLzggZKTeilLWgLGc51a3gkuCjKQ7DO7cXZotgJ3kLqzC6hmex1SZnSXTWYHcrj8LJjWTk0PHoZ8VqIdCOKayPykcpuQxAokJaG1dGybYj4gw4K5u6PKTasSbjXKgnIDlA8VvUdo-pzonraBY2bsH7HAl78mKSHZpgIcuHjq9jXSpZSLixRlveKYQUXhQVhL6GPobXAAb7BbNeyvNUs4qfRg3OnELLj5hqH9eQZqCnoBwR9lYcQxuVXeBzc8kMF8yXY4yNJ5oGiUzP_aaf_waTRGJib5_Ad3P_vbCuaYxzeNpbU0eUMPAOKh7Yw1YErgtoXyuYlPLzc10_xo_5A91zkQL_AgAA)):

```svelte
<svelte:boundary>
	<FlakyComponent />

	{#snippet failed(error, reset)}
		<button onclick={reset}>oops! try again</button>
	{/snippet}
</svelte:boundary>
```

> [!NOTE]
> As with [snippets passed to components](snippet#Passing-snippets-to-components), the `failed` snippet can be passed explicitly as a property...
>
> ```svelte
> <svelte:boundary {failed}>...</svelte:boundary>
> ```
>
> ...or implicitly by declaring it directly inside the boundary, as in the example above.

### `onerror`

If an `onerror` function is provided, it will be called with the same two `error` and `reset` arguments. This is useful for tracking the error with an error reporting service...

```svelte
<svelte:boundary onerror={(e) => report(e)}>
	...
</svelte:boundary>
```

...or using `error` and `reset` outside the boundary itself:

```svelte
<script>
	let error = $state(null);
	let reset = $state(() => {});

	function onerror(e, r) {
		error = e;
		reset = r;
	}
</script>

<svelte:boundary {onerror}>
	<FlakyComponent />
</svelte:boundary>

{#if error}
	<button onclick={() => {
		error = null;
		reset();
	}}>
		oops! try again
	</button>
{/if}
```

If an error occurs inside the `onerror` function (or if you rethrow the error), it will be handled by a parent boundary if such exists.