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ElementsKit

Lit

A custom element built with elements-kit is just class extends HTMLElement registered via customElements.define. Lit renders it fine in html template literals — no configuration needed. Because Lit uses tagged templates rather than JSX, there is no IntrinsicElements to augment. The main integration point is getting typed element references in @query decorators and this.renderRoot.querySelector calls.

// 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.

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

Typed element queries

Augmenting HTMLElementTagNameMap narrows the return type of @query, this.renderRoot.querySelector, and document.querySelector:

x-counter.ts
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);


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

Inside a Lit host element:

import { LitElement, html } from "lit";
import { customElement, query } from "lit/decorators.js";
import "./x-counter"; // brings in the HTMLElementTagNameMap augmentation


@customElement("my-host")
export class MyHost extends LitElement {
  // @query narrows to XCounter because of HTMLElementTagNameMap
  @query("x-counter") counter!: XCounter;


  render() {
    return html`<x-counter></x-counter>`;
  }


  updated() {
    this.counter.count = 10; // typed
  }
}

PropertiesOf as a type annotation

PropertiesOf<typeof XCounter> is useful when you need to describe the settable surface of an elements-kit element — for example, inside a factory or test helper:

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