hello-element.js

class HelloElement extends HTMLElement {
  connectedCallback() {
    this.innerHTML = this.render('Nettleser: Hei');
  }
  render(text) {
    return `<h1> ${text} </h1>`;
  }
}
customElements.define('hello-element', HelloElement);

server.js

global.HTMLElement = class EmptyHTMLElement {};

const helloElement = new HelloElement();
const serverHtml = `
<hello-element>
  ${helloElement.render('Server: Hei')}
</hello-element>`;

function renderElement(elementConstructor, tagName, props) {
  const element = new elementConstructor();
  const html = element.render(props);

  return `<${tagName}> ${html} </${tagName}>`;
}

const html = renderElement(HelloElement, 'hello-element', 'Server: Hei');