# Svelte

A custom element built with elements-kit is just `class extends HTMLElement` registered via `customElements.define`. Svelte renders it fine at runtime in both Svelte 4 and Svelte 5 (runes mode), but you need a small augmentation to get typed props, narrowed `bind:this` refs, and editor autocomplete.

```ts
// shared element — built once, used anywhere
import { defineElement } from "elements-kit/custom-elements";
import { reactive } from "elements-kit/signals";

export class XCounter extends HTMLElement {
  @reactive() count = 0;
}

defineElement("x-counter", XCounter);
```

## Source the prop shape from the class

Three type helpers exported from `elements-kit/custom-elements` read the raw platform surface off the class. All three take the constructor (`typeof XCounter`); `PropertiesOf` also accepts an instance type.

| Helper                          | Shape                                                                                                            | Use when                                                                                 |
| ------------------------------- | ---------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------- |
| `PropertiesOf<typeof XCounter>` | Public instance fields, all optional — the `HTMLElement` surface is dropped; `Slot`-typed fields accept a `Node` | Typing property assignments — prop bindings, wrappers, helper functions                  |
| `AttributesOf<typeof XCounter>` | Every key of `static [ATTRIBUTES]`, valued `string \| null`                                                      | Typing the raw HTML attribute surface — `setAttribute`, server-rendered markup           |
| `EventsOf<typeof XCounter>`     | The `static events` map, verbatim — event name to event type                                                     | Typing `addEventListener`, framework event bindings, `HTMLElementEventMap` augmentations |

Namespaced JSX props (`on:*`, `class:*`) don't exist in other
frameworks. Slot content is set at property level (they're regular
properties — assign to fill, read to inspect; `PropertiesOf` types those
keys as content), and events attach with `addEventListener`, typed via
`EventsOf`.

## Augment Svelte's `SvelteHTMLElements`

```ts
import type { HTMLAttributes } from "svelte/elements";
import type { PropertiesOf } from "elements-kit/custom-elements";
import type { XCounter } from "./x-counter";

declare module "svelte/elements" {
  export interface SvelteHTMLElements {
    "x-counter": HTMLAttributes<XCounter> & PropertiesOf<typeof XCounter>;
  }
}

// <x-counter count={5} bind:this={el} />  — el: XCounter
```

`HTMLAttributes<XCounter>` provides the standard HTML attribute surface (`class`, `id`, event handlers, …) and ensures `bind:this` resolves to `XCounter`.

## Co-locate the augmentation with the class

Put the `declare module "svelte/elements"` block in the same file as the element class. Importing the class then brings the augmentation along — consumers don't have to remember to write it themselves.

```ts
// x-counter.ts
import { defineElement, type PropertiesOf } from "elements-kit/custom-elements";
import { reactive } from "elements-kit/signals";
import type { HTMLAttributes } from "svelte/elements";

export class XCounter extends HTMLElement {
  @reactive() count = 0;
}

defineElement("x-counter", XCounter);

declare module "svelte/elements" {
  export interface SvelteHTMLElements {
    "x-counter": HTMLAttributes<XCounter> & PropertiesOf<typeof XCounter>;
  }
}
```

Make sure the file is referenced in your `tsconfig.json`'s `include` array so TypeScript picks up the augmentation.

## Typed `document.querySelector` and `createElement`

Independent of any framework, augmenting the global `HTMLElementTagNameMap` gives you typed lookups in plain DOM code:

```ts
declare global {
  interface HTMLElementTagNameMap {
    "x-counter": XCounter;
  }
}

const el = document.querySelector("x-counter"); // XCounter | null
```

Worth doing alongside the Svelte augmentation.

***

## See also

* [Custom Elements overview](/custom-elements)
* [React integration](/integrations/react)
* [Solid integration](/integrations/solid)
* [Vue integration](/integrations/vue)
* [Angular integration](/integrations/angular)
* [Lit integration](/integrations/lit)
* [Astro integration](/integrations/astro)
* [Qwik integration](/integrations/qwik)