dynamic_docs.js

function onOpen() {
  DocumentApp.getUi().createMenu('Demo')
      .addItem('Select Spreadsheet', 'selectSpreadsheet')
      .addItem('Update Data', 'updateData')
      .addToUi();
}

function selectSpreadsheet() {
  var result = DocumentApp.getUi().prompt('Enter the ID of the spreadsheet:');
  var spreadsheetId = result.getResponseText();
  PropertiesService.getDocumentProperties().setProperty('spreadsheetId', spreadsheetId);
}

function updateData() {
  var doc = DocumentApp.getActiveDocument();
  var spreadsheetId = PropertiesService.getDocumentProperties().getProperty('spreadsheetId');
  var ss = SpreadsheetApp.openById(spreadsheetId);
  findNewTags(doc);
  doc.getNamedRanges().forEach(function(namedRange) {
    // namedRange = doc.getNamedRanges()[];
    var name = namedRange.getName();
    var value = ss.getRangeByName(name).getValue().toString();
    var rangeElement = namedRange.getRange().getRangeElements()[0];
    if (rangeElement.isPartial()) {
      var text = rangeElement.getElement().asText();
      text.insertText(rangeElement.getEndOffsetInclusive() + 1, value);
      text.deleteText(rangeElement.getStartOffset(), rangeElement.getEndOffsetInclusive());
    } else {
      throw 'TODO';
    }
  });
}

function findNewTags(doc) {
  var body = doc.getBody();
  var match = null;
  while (match = body.findText('{{.*?}}', match)) {
    var element = match.getElement();
    var range = doc.newRange();
    var name;
    if (match.isPartial()) {
      var text = match.getElement().asText();
      range.addElement(text, match.getStartOffset(), match.getEndOffsetInclusive());
      name = text.getText().substring(match.getStartOffset() + 2, match.getEndOffsetInclusive() - 1);
    } else {
      throw 'TODO';
    }
    doc.addNamedRange(name, range);
  }
}