copy titles into all documents in a folder

Code.gs

function copyTitles() {
  //Replace this variable with the folder id
  //you can get the id from the URL
  var folderId = 'REPLACE_WITH_FOLDER_ID';
  
  var folder = DriveApp.getFolderById(folderId);
  
  var files = folder.getFilesByType(MimeType.GOOGLE_DOCS);
  
  //this will iterate over every single DOCUMENT in that folder
  //it will ignore spreadsheets, forms, and other files
  //you can modify the mimetype above to change that behavior
  while(files.hasNext()) {
  
    var file = files.next();
    
    var fileId = file.getId();
    
    //this script takes the file name and inserts it into the very
    //top of the document.
    var fileName = file.getName();
    
    DocumentApp.openById(fileId).getBody()
      .insertParagraph(0, fileName)
      .setAlignment(DocumentApp.HorizontalAlignment.CENTER)
      .setHeading(DocumentApp.ParagraphHeading.HEADING1);
  
  }
}