documentTemplates/Code.ts

function onOpen() {
  const ui = SpreadsheetApp.getUi();
  ui.createMenu('test')
    .addItem('test1', 'test')
    .addToUi();
}

function copyElement(
  source_element: GoogleAppsScript.Document.Element,
  dest: GoogleAppsScript.Document.Body,
  index?: number
) {
  const element = source_element.copy();
  // based on https://stackoverflow.com/questions/6783819/google-app-script-copy-document-page
  if (element.getType() == DocumentApp.ElementType.PARAGRAPH)
    if (index) dest.insertParagraph(index, element.asParagraph());
    else dest.appendParagraph(element.asParagraph());
  else if (element.getType() == DocumentApp.ElementType.TABLE)
    if (index) dest.insertTable(index, element.asTable());
    else dest.appendTable(element.asTable());
  else if (element.getType() == DocumentApp.ElementType.LIST_ITEM)
    if (index) dest.insertListItem(index, element.asListItem());
    else dest.appendListItem(element.asListItem());
  else if (element.getType() == DocumentApp.ElementType.INLINE_IMAGE)
    if (index) dest.insertImage(index, element.asInlineImage());
    else dest.appendImage(element.asInlineImage());
  else
    throw new Error(
      "According to the doc this type couldn't appear in the body: " +
        element.getType()
    );
}

function copyBody(
  source: GoogleAppsScript.Document.Body,
  dest: GoogleAppsScript.Document.Body,
  index?: number
) {
  var totalElements = source.getNumChildren();
  for (var j = 0; j < totalElements; ++j) {
    if (index) copyElement(source.getChild(j), dest, index + j);
    else copyElement(source.getChild(j), dest);
  }
}

function test() {
  const cell = SpreadsheetApp.getActive().getCurrentCell();
  const template = DocumentApp.openById(
    '1V0uMuM80BGpjpdt1AmuzlU97tDI_u-y2rOfdl4tkqmc'
  );

  const source_doc = DocumentApp.openById(
    '1tB9--ilbfDixuQAEMk0_D6KqNFToQzs-au6NiotCH5M'
  );
  const out_doc = template; // TODO: make a copy of template, and write to that

  const insert_point = out_doc
    .getBody()
    .findText('{{body}}')
    ?.getElement();

  if (!insert_point) {
    throw new Error("Could not find insert point '{{body}}'");
  }

  // find the parent element that is a direct descendant of the body
  let parent = insert_point;
  while (parent.getParent().getType() != DocumentApp.ElementType.BODY_SECTION) {
    parent = parent.getParent();
  }
  const index = out_doc.getBody().getChildIndex(parent);

  // insert with index 0 is an append, for some reason
  copyBody(source_doc.getBody(), out_doc.getBody(), index + 1);
  out_doc.getBody().removeChild(parent);
}