# Marko

A custom element built with elements-kit is just `class extends HTMLElement` registered via `customElements.define`. Marko renders it fine at runtime — custom elements with a hyphen in the tag name are passed through as-is. TypeScript support for Marko template types is provided by [`@marko/language-tools`](https://github.com/marko-js/language-tools) and is evolving; the most reliable typing surface today is `HTMLElementTagNameMap` for DOM queries.

```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 |

## Use the element in a template

Import the element registration before the template renders client-side. In a Marko component, put the import in `<script>` or the component's class body:

```marko
// x-counter.marko
import "./x-counter"; // registers the custom element

<x-counter count=5 />
```

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

Augmenting the global `HTMLElementTagNameMap` gives you typed DOM lookups in any TypeScript file — Marko template logic, server routes, or client scripts:

```ts
// types/elements.d.ts
import type { XCounter } from "./src/x-counter";

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

Make sure this file is covered by your `tsconfig.json`'s `include` array.

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

`PropertiesOf<typeof XCounter>` is useful as a type annotation when writing helper functions that operate on the element:

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

function applyProps(el: XCounter, props: PropertiesOf<typeof XCounter>) {
  Object.assign(el, props);
}
```

***

## See also

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