legendary-doc-site/apps/app/assets/js/admin/content-editor.js

import { ready } from "../utils";
import SimpleMDE from "simplemde";
import "simplemde/dist/simplemde.min.css";
import "../../css/content-editor-overrides.css";

const requestPreview = (plainText, previewContainer) => {
  let request = new XMLHttpRequest();
  const postForm = previewContainer.closest("form");
  let formData = new FormData(postForm);

  formData.set("post[content]", plainText);

  request.addEventListener("load", function (event) {
    previewContainer.innerHTML = event.target.responseText;
  });

  request.open("POST", "/posts/preview", true);

  request.send(formData);
};

ready(() => {
  document.querySelectorAll("[data-simplemde]").forEach((el) => {
    new SimpleMDE({
      element: el,
      previewRender: (plainText, previewContainer) => {
        requestPreview(plainText, previewContainer);

        return previewContainer.innerHTML;
      },
    });
  });
});