attempt.scala

@js.native
@JSGlobal("BlotEmbed")
class BlotEmbed extends js.Object {
  var blotName: String = js.native
  var tagName: String = js.native
  var className: String = js.native

  def create(data: js.Object): html.Element = js.native

  def setDataValues(element: html.Element, data: js.Object): html.Element = js.native

  def value(domNode: html.Element): js.Object = js.native
}

object TestBlot extends BlotEmbed {
  @JSExportStatic
  override def create(data: js.Object): Element = {
    val node = super.create(data)
    val block = document.createElement("span")
    block.classList.add("ql-testing")
    block.innerHTML = data.asInstanceOf[Value].value
    node.appendChild(block)
    setDataValues(node, data)
  }

  @JSExportStatic
  override def setDataValues(element: Element, data: js.Object): Element = {
    // TODO: implement
    element
  }

  @JSExportStatic
  override def value(domNode: Element): js.Object = js.Object()
}

trait Value {
  def value: String
}

blot-embed.js

    const BlotEmbed = Quill.import("blots/embed");

    class TestBlot extends BlotEmbed {
        static create(data) {
            const node = super.create();
            const block = document.createElement("span");
            block.className = "ql-testing";
            block.innerHTML = data.value;
            node.appendChild(block);
            return TestBlot.setDataValues(node, data);
        }

        static setDataValues(element, data) {
            const domNode = element;
            Object.keys(data).forEach(key => {
                domNode.dataset[key] = data[key];
            });
            return domNode;
        }

        static value(domNode) {
            return domNode.dataset;
        }
    }

    TestBlot.blotName = "testing";
    TestBlot.tagName = "span";
    TestBlot.className = "testing";
    Quill.register(TestBlot);