before.ts

import { 
  LitElement, 
  customElement, 
  html, 
  css, 
  property 
} from "lit-element";

const blockStyle = css`:host { display: block; }`

@customElement("hello-world")
export class HelloWorld extends LitElement {
  @property()
  who: string = "noone";

  static get styles() {
    return [blockStyle];
  }

  render() {
    return html`
      Hello ${this.who}!
    `;
  }
}

@customElement("greet-em")
export class GreetEm extends LitElement {
  render() {
    return html`
      <div>
        ${["george", "john"].map(
          n =>
            html`
              <hello-world who="${n}"></hello-world>
            `
        )}
      </div>
    `;
  }
}