todo-lit/src/components/click-counter.ts

import {LitElement, html, css} from 'lit-element';
import {customElement, property} from 'lit/decorators.js';

@customElement('click-counter')
export class ClickCounter extends LitElement {
  static styles = css`
    :host {
      display: block;
    }
  `;

  @property({type: Number})
  count = 0;

  render() {
    return html`
      <button @click=${this._onClick}>
        Click count: ${this.count}    
      </button>
    `;
  }

  private _onClick() {
    this.count++;
  }
}

declare global {
  interface HTMLElementTagNameMap {
    'click-counter': ClickCounter;
  }
}