I believe that on the server side, everything is purely synchronous with Meteor, thanks to MiniMongo. Thus, an asynchronous callback is not required, or, to be more precise, Meteor does not require waiting for the update callback from the server to proceed to the next instruction, thanks to MiniMongo, which responds synchronously.
I did not check everything I just said while studying the source code, but I canβt imagine how it can work differently.
EDIT
Not yet plunged into the source code, but this section of the Meteor manual seems to be moving as I thought.
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