Issue summary

I hesitated to split this into 2 issues because they are rather tightly coupled

1. Issue 1: When prefix is not set, transferManager.downloadManyFiles does not actually write files locally. This is because destination is undefined.
2. Related issue 2: When prefix is set, file.download(...) attempts to write the file, but is unable to recursively create directory structure on local machine.

Proposed draft PR

#2199

Standard questions

1. Is this a client library issue or a product issue? - client library
2. Did someone already solve this? - it hasn't been asked yet in the Issue or Stack overflow
3. Do you have a support contract? - At work yes, but this is a personal contribution outside work

Environment details

• OS: macOS Catalina
• Node.js version: v16.15.0
• npm version: 8.5.5
• @google-cloud/storage version: 6.10.1

Steps to reproduce

Issue 1:

1. Implement code example for 'downloadManyFiles' without prefix

async function downloadDirectory({ bucketId, sourceDirectory }) {
  const storage = new Storage();
  const transferManager = new TransferManager(storage.bucket(bucketId));
  const options = {};

  async function downloadFolderWithTransferManager() {
    await transferManager.downloadManyFiles(sourceDirectory, options);
  }
  const res = await downloadFolderWithTransferManager();
}

2. Running the function returns no error. However, no files are written locally, and a well-placed console.warn shows that this is because the destination is undefined. The draft PR contains a suggestion to always generate the destination.

Issue 2:

1. Implement same example for 'downloadManyFiles', except we now set prefix

async function downloadDirectory({ bucketId, sourceDirectory }) {
  const storage = new Storage();
  const transferManager = new TransferManager(storage.bucket(bucketId));

  const options = {
    prefix: "hello",
  };
  async function downloadFolderWithTransferManager() {
    await transferManager.downloadManyFiles(sourceDirectory, options);
  }
  const res = await downloadFolderWithTransferManager();
}

2. Running the function returns

ENOENT: no such file or directory, open 'hello/test-folder/'

This is due to the file stream at

Thanks, it is my first issue and fork of the repo! Open to feedback. I've already signed the CLA, but kept the PR as draft until the codeowner reviews the issue/intention of the existing design.