Svelte provides a best effort of all the HTML DOM types that exist. Sometimes you may want to use experimental attributes or custom events coming from an action. In these cases, TypeScript will throw a type error, saying that it does not know these types. If it's a non-experimental standard attribute/event, this may very well be a missing typing from our[HTML typings](https://github.com/sveltejs/svelte/blob/main/packages/svelte/elements.d.ts). In that case, you are welcome to open an issue and/or a PR fixing it.

In case this is a custom or experimental attribute/event, you can enhance the typings like this:

declarenamespacesvelteHTML {

interfaceIntrinsicElements {

'my-custom-element': { someattribute:string;'on:event': (e:CustomEvent<any>)=>void };

}

interfaceHTMLAttributes<T> {

onbeforeinstallprompt?: (event:any)=>any;

mycustomattribute?:any;// You can replace any with something more specific if you like

}

}

Then make sure that`d.ts` file is referenced in your`tsconfig.json`. If it reads something like`"include": ["src/**/*"]` and your`d.ts` file is inside`src`, it should work. You may need to reload for the changes to take effect.

You can also declare the typings by augmenting the`svelte/elements` module like this:

In case this is a custom or experimental attribute/event, you can enhance the typings by augmenting the`svelte/elements` module like this:

import {HTMLButtonAttributes }from'svelte/elements';

declaremodule'svelte/elements' {

exportinterfaceSvelteHTMLElements {

'custom-button':HTMLButtonAttributes;

}

exportinterfaceHTMLAttributes<T> {

globalattribute?:string;

}

exportinterfaceHTMLButtonAttributes {

veryexperimentalattribute?:string;

}

}

export {};// ensure this is not an ambient module, else types will be overridden instead of augmented

Then make sure that the`d.ts` file is referenced in your`tsconfig.json`. If it reads something like`"include": ["src/**/*"]` and your`d.ts` file is inside`src`, it should work. You may need to reload for the changes to take effect.

If side-effects are unavoidable, use[`$effect`]($effect) instead.

If a[`<svelte:boundary>`](https://svelte.dev/docs/svelte/svelte-boundary) has an`onerror` function, it must not call the provided`reset` function synchronously since the boundary is still in a broken state. Typically,`reset()` is called later, once the error has been resolved.

If it's possible to resolve the error inside the`onerror` callback, you must at least wait for the boundary to settle before calling`reset()`, for example using[`tick`](https://svelte.dev/docs/svelte/lifecycle-hooks#tick):

To resolve this, ensure you're comparing values where both values were created with`$state(...)`, or neither were. Note that`$state.raw(...)` will _not_ create a state proxy.

A`<svelte:boundary>``reset`function only resets the boundary the first time it is called

```

When an error occurs while rendering the contents of a [`<svelte:boundary>`](https://svelte.dev/docs/svelte/svelte-boundary), the `onerror` handler is called with the error plus a `reset` function that attempts to re-render the contents.

This`reset`function should only be called once. After that, it has no effect — in a case like this, where a reference to `reset` is stored outside the boundary, clicking the button while `<Contents />` is rendered will _not_ cause the contents to be rendered again.

```svelte

<script>

let reset;

</script>

<button onclick={reset}>reset</button>

<svelte:boundary onerror={(e,r) => (reset = r)}>

{#snippetfailed(e)}

<p>oops! {e.message}</p>

{/snippet}

If side-effects are unavoidable, use[`$effect`]($effect) instead.

If a[`<svelte:boundary>`](https://svelte.dev/docs/svelte/svelte-boundary) has an`onerror` function, it must not call the provided`reset` function synchronously since the boundary is still in a broken state. Typically,`reset()` is called later, once the error has been resolved.

If it's possible to resolve the error inside the`onerror` callback, you must at least wait for the boundary to settle before calling`reset()`, for example using[`tick`](https://svelte.dev/docs/svelte/lifecycle-hooks#tick):

To resolve this, ensure you're comparing values where both values were created with`$state(...)`, or neither were. Note that`$state.raw(...)` will _not_ create a state proxy.

A`<svelte:boundary>``reset`function only resets the boundary the first time it is called

```

When an error occurs while rendering the contents of a [`<svelte:boundary>`](https://svelte.dev/docs/svelte/svelte-boundary), the `onerror` handler is called with the error plus a `reset` function that attempts to re-render the contents.

This`reset`function should only be called once. After that, it has no effect — in a case like this, where a reference to `reset` is stored outside the boundary, clicking the button while `<Contents />` is rendered will _not_ cause the contents to be rendered again.

```svelte

<script>

let reset;

</script>

<button onclick={reset}>reset</button>

<svelte:boundary onerror={(e,r) => (reset = r)}>

{#snippetfailed(e)}

<p>oops! {e.message}</p>

{/snippet}