Google Docs - Replace a Watermark on a directory of Docs

replace-watermarks.js

/// Set these three variables.
/// They should exist so the paths are:
/// sourceDirName
///   watermark-template (can be renamed, change the variable)
///   lots of Docs files
/// | destDirName
///   | new files will be here
/// That is, sourceDirName is a directory that contains watermark-template, destDirName, and some number of documents.
/// Please copy files into the sourceDirName directory rather than trusting this to process all at once or to not break things.
/// Verify new files.

/// In the source dir select: New >> More >> Google Apps Script
/// Paste into the editor.
/// Save Project
/// Click Run

/// Source: Grok saved a lot of time writing this, it might not be ideal, but it works.

var sourceDirName = "original";
var destDirName = "watermarked";
var waterMarkTemplateName = 'watermark-template'

function getFileIdByName(name) {
  var files = DriveApp.getFilesByName(name);
  if (files.hasNext()) {
    var file = files.next();
    Logger.log(file.getId()); // Logs the file ID
    return file.getId();
  }
}

function copyContentToWatermarkedDocForFolder() {
  // ID of the folder containing the source documents
  var sourceFolderId = DriveApp.getFoldersByName(sourceDirName).next().getId();
  console.log(`Using ${sourceDirName}: ${sourceFolderId}`)
  var destFolderId = DriveApp.getFoldersByName(destDirName).next().getId();
  console.log(`Output ${destDirName}: ${destFolderId}`)
  
  // ID of the template document with the watermark (destination)
  var watermarkedTemplateId = getFileIdByName(waterMarkTemplateName);
  if (watermarkedTemplateId === undefined) {
    throw new Error(`Template file was not found, or not named ${waterMarkTemplateName}`)
  }
  console.log(`Using ${watermarkedTemplateId}`)

  // Get the folder
  var sourceFolder = DriveApp.getFolderById(sourceFolderId);
  var destinationFolder = DriveApp.getFolderById(destFolderId); // Assuming you want to save in the same folder
  
  // Open the watermarked template document
  var templateFile = DriveApp.getFileById(watermarkedTemplateId);
  var templateDoc = DocumentApp.openById(watermarkedTemplateId);

  // Iterate through all files in the source folder
  var files = sourceFolder.getFilesByType(MimeType.GOOGLE_DOCS);
  
  while (files.hasNext()) {
    var file = files.next();
    var fileName = file.getName();
    if (fileName == waterMarkTemplateName) {
      continue;
    }
    
    // Check if a watermarked copy already exists
    var existingWatermarkedFile = destinationFolder.getFilesByName(fileName);
    if (existingWatermarkedFile.hasNext()) {
      Logger.log('A watermarked copy of ' + fileName + ' already exists. Skipping.');
      continue;
    }

    try {
      var sourceDoc = DocumentApp.openById(file.getId());
      var sourceBody = sourceDoc.getBody();
      
      // Create a new document from the template with watermark using the File object
      var newFile = templateFile.makeCopy(fileName + ' - Watermarked', destinationFolder);
      var newDoc = DocumentApp.openById(newFile.getId());
      var newDocBody = newDoc.getBody();
      
      // Clear any existing content in the new document except for the watermark
      newDocBody.clear();

      // Copy all content from the source document to the new document
      var numChildren = sourceBody.getNumChildren();
      for (var i = 0; i < numChildren; ++i) {
        var element = sourceBody.getChild(i).copy();
        var type = element.getType();
        
        switch(type) {
          case DocumentApp.ElementType.PARAGRAPH:
            newDocBody.appendParagraph(element);
            break;
          case DocumentApp.ElementType.TABLE:
            newDocBody.appendTable(element);
            break;
          case DocumentApp.ElementType.LIST_ITEM:
            newDocBody.appendListItem(element);
            break;
          case DocumentApp.ElementType.IMAGE:
            newDocBody.appendImage(element);
            break;
          case DocumentApp.ElementType.INLINE_IMAGE:
            newDocBody.appendInlineImage(element);
            break;
          case DocumentApp.ElementType.HORIZONTAL_RULE:
            newDocBody.appendHorizontalRule(element);
            break;
          case DocumentApp.ElementType.PAGE_BREAK:
            newDocBody.appendPageBreak(element);
            break;
          case DocumentApp.ElementType.HEADING:
            newDocBody.appendParagraph(element); // Treats heading as a paragraph
            break;
          case DocumentApp.ElementType.FOOTNOTE:
            newDocBody.appendFootnote(element);
            break;
          // Add other element types as needed
          default:
            Logger.log('Unhandled element type: ' + type);
        }
      }

      // Optionally, save the new document
      newDoc.saveAndClose();
      
      // Log the URL of the new document for reference
      Logger.log('Created new document with watermark: ' + newDoc.getUrl());
    } catch (error) {
      // Log errors for each document if any arise
      Logger.log('Error processing document ' + fileName + ': ' + error);
    }
  }
}

At this time, Google's API doesn't provide a way to replace watermarks programmatically. This copies the contents to a copy of a template file.