How can I discriminate between file and folder in

Question

I know a folder actually is a file with a special mimeType application/vnd.google-apps.folder in Google Drive. But documentation about DriveApp Folder class doesn't contain a method getMimeType() like DriveApp File class does.

How can find out in a function if an argument of that function refers to a folder or to a (normal) file?

function displayResults(fileOrFolder)
{
  var id   = fileOrFolder.getId();
  var name =  fileOrFolder.getName();

  var mimeType = fileOrFolder.getMimeType();  // works if file; error if folder

  if (fileOrFolder == <Class File>)
  {
     <do something>
  }
  else if (fileOrFolder == <Class Folder>)
  {
     <do something else>
  }
}

Answer 1

Get the file object associated to the ID. getMimeType() will work:

var mt = DriveApp.getFileByID(fileOrFolder.getId()).getMimeType();
if (mt == MimeType.FOLDER) {
  // it's a folder
} else {
  // it's a file.
}

Answer 2

Look if the method exists. If obj.getMimeType===undefined its a folder.
or, to make it more robust in case the api changes (adds the method to folders), use
if (obj.getMimeType===undefined || obj.getMimeType.toLowerCase() =="application/vnd.google-apps.folder")

Answer 3

I don't understand why you say that Zig's answer does not work... All the tests I made were positive.

example of how I tested it (test ID are shared publicly and so should work for anybody):

function test(){
  var item = DriveApp.getFileById('0B3qSFd3iikE3MS0yMzU4YjQ4NC04NjQxLTQyYmEtYTExNC1lMWVhNTZiMjlhMmI');
  testDriveFolders(item);
  var item = DriveApp.getFileById('1fXxLHuDlySZ6rOQjTsZdUl6rnw14sa17Q0ZTZ0HoGk1jbuHX-eLq1dyf');
  testDriveFolders(item);
}

function testDriveFolders(obj){
  try{var mimeType = obj.getMimeType()}catch(e){mimeType = 'folder'};
  var name =  obj.getName();
  if (mimeType =="application/vnd.google-apps.folder" || mimeType == 'folder'){
    Logger.log(name+' is a folder');
  }else{
        Logger.log(name+' is of the type '+mimeType);
  }
}