function getFolder_(folderName) {

  // Gets the Drive folder where the form is located.
  const ssId = FormApp.getActiveForm().getId();
  const parentFolder = DriveApp.getFileById(ssId).getParents().next();

  // Iterates through the subfolders to check if folder already exists.
  // The script checks for the folder name specified in the APP_FOLDER_NAME variable.
  const subFolders = parentFolder.getFolders();
  while (subFolders.hasNext()) {
    let folder = subFolders.next();

    // Returns the existing folder if found.
    if (folder.getName() === folderName) {
      return folder;
    }
  }
  // Creates a new folder if one doesn't already exist.
  return parentFolder.createFolder(folderName)
    .setDescription(`Created by ${APP_TITLE} application to store uploaded files.`);
}
Returns a folder to store uploaded files in the same location
in Drive where the form is located. First, it checks if the folder
already exists, and creates it if it doesn't.

@param {string} folderName - Name of the Drive folder. 
@return {object} Google Drive Folder