Is socket.io emit callback appropriate?

Question

Recently I have been messing around with socket.io and found this interesting thing, that I can have emit function callback like this.

I start emitting on client side li

Answer 1

It's perfectly legal.

Those callbacks are called 'acknowledgement functions' and are summarily mentioned in the Wiki and described a bit more in detail on the NPM page ('Getting acknowledgements').

EDIT: more recent documentation can be found here.