function getFolder_(folderName) {

  // Gets the primary folder for the application.
  const parentFolder = getApplicationFolder_();

  // Iterates subfolders to check if folder already exists.
  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(`Supporting folder created by ${APP_TITLE}.`);
}
Returns folder named in folderName parameter. 
Checks if folder already exists,  creates it if it doesn't.

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