function AddClassToAllHeading1(editor) {
  // Both the data and the editing pipelines are affected by this conversion.
  editor.conversion.for("downcast").add((dispatcher) => {
    // Headings are represented in the model as a "heading1" element.
    // Use the "low" listener priority to apply the changes after the headings feature.
    dispatcher.on(
      "insert:listItem",
      (evt, data, conversionApi) => {
        const viewWriter = conversionApi.writer;

        viewWriter.addClass(
          "list-item",
          conversionApi.mapper.toViewElement(data.item)
        );
      },
      { priority: "low" }
    );
  });
}

ClassicEditor.create(document.querySelector(".editor"), {
  toolbar: {
    items: [
      "heading",
      "|",
      "bold",
      "italic",
      "bulletedList",
      "numberedList",
      "link",
      "blockQuote",
      "|",
      "undo",
      "redo",
      "|"
    ]
  },
  heading: {
    options: [
      {
        model: "paragraph",
        title: "Paragraph",
        class: "ck-heading_paragraph"
      },
      {
        model: "heading2",
        view: "h2",
        title: "Heading 2",
        class: "ck-heading_heading2"
      },
      {
        model: "heading3",
        view: "h3",
        title: "Heading 3",
        class: "ck-heading_heading3"
      },
      {
        model: "heading4",
        view: "h4",
        title: "Heading 4",
        class: "ck-heading_heading4"
      },
      {
        model: "heading2_class",
        view: {
          name: "p",
          classes: "h2"
        },
        title: "Heading 2 (class)",
        class: "ck-heading_heading2",
        // It needs to be converted before the standard 'heading2'.
        priority: "high"
      },
      {
        model: "heading3_class",
        view: {
          name: "p",
          classes: "h3"
        },
        title: "Heading 3 (class)",
        class: "ck-heading_heading3",
        // It needs to be converted before the standard 'heading2'.
        priority: "high"
      },
      {
        model: "heading4_class",
        view: {
          name: "p",
          classes: "h4"
        },
        title: "Heading 4 (class)",
        class: "ck-heading_heading4",
        // It needs to be converted before the standard 'heading2'.
        priority: "high"
      }
    ]
  },
  extraPlugins: [AddClassToAllHeading1],
  language: "ru"
})
  .then((editor) => {
    window.editor = editor;
  })
  .catch((error) => {
    console.error("Oops, something gone wrong!");
    console.error(
      "Please, report the following error in the https://github.com/ckeditor/ckeditor5 with the build id and the error stack trace:"
    );
    console.warn("Build id: qx0uru7ua4iy-lg3xgz2xd0rt");
    console.error(error);
  });