How to check if DriveApp.getFolderById() returns a folder?

Question

When I use DriveApp.getFolderById(someId) where someId belongs to a file and not a folder, the method returns the file instead of erring out. How can I ensure that the returned object is indeed a folder?

Right now, the only solution I can think of is with try/catch:

try {
  const testFile = someFile.makeCopy('test', folder);
  testFile.setTrashed(true);
} catch (err) {
  return 'The folder is not really a folder';  
}

Is there a cleaner solution?

Answer 1

In your situation, for example, how about checking the mimeType as follows?

Sample script:

const id = "###"; // Please set the file ID and folder ID.

const file = DriveApp.getFileById(id);
if (file.getMimeType() == MimeType.FOLDER) {
  console.log(`This ID (${id}) is a folder.`);
  const folder = DriveApp.getFolderById(id);

  // Do something.

} else {
  console.log(`This ID (${id}) is not a folder.`);
}

• When this script is run, by checking the mimeType of id, you can see whether the ID is the folder.

Reference:

• getMimeType()

Added:

From your following reply,

A folder doesn't have a getMimeType function to call, so I guess a check like if (!folder.getMimeType) that a returns true indicates that it's a folder.

If you want to check using const folder = DriveApp.getFolderById(id), how about the following sample script?

Sample script:

const id = "###"; // Please set the file ID and folder ID.

const folder = DriveApp.getFolderById(id);
if (folder.getUrl().split("/")[4] == "folders") {
  console.log(`This ID (${id}) is a folder.`);

  // In this case, you can directly use the variable of "folder".
  // Do something.

} else {
  console.log(`This ID (${id}) is not a folder.`);
}

• In this sample script, by checking the URL of the object, it is checked whether id is the folder. For example, when id is Spreadsheet, the retrieved URL is like https://docs.google.com/spreadsheets/d/###/edit?usp=drivesdk. This script uses this situation.