Move done() to transaction.done();

From the docs:

dataset.runInTransaction(function(transaction, done) {
  // From the `transaction` object, execute dataset methods as usual.
  // Call `done` when you're ready to commit all of the changes.
  transaction.get(dataset.key('Company', 123), function(err, entity) {
    if (err) {
      transaction.rollback(done);
      return;
    }

    done();
  });
}, function(err) {});

Any reason why done is separate from transaction?
Can we replace done with transaction.done?

Also, what does calling rollback with the done function do? Shouldn't it do this by default? It could if we can just call this.done() from within Transaction.prototype.rollback() ?

Any reason why done is separate from transaction?
Can we replace done with transaction.done?

It's common to see flow control callbacks come as their own variable. I'd like to stay with done.

Also, what does calling rollback with the done function do? Shouldn't it do this by default? It could if we can just call this.done() from within Transaction.prototype.rollback() ?

It could do it by default, but the user should understand the flow. "When rollback is done, this function is exiting. My callback to runInTransaction will be next."