So at work we're working on a bunch of node modules that will eventually be 
published as open-source and I'm in favor of callbacks and two of my 
co-workers are in favor of promises. We've discussed supporting both API 
interfaces and I was curious what the general consensus of the community 
was with respect to supporting both and the best way to name functions and 
methods to support both.

That being said, there are three obvious choices:

(a) two function types: (1) synchronous functions; and (2) async functions 
that return promises but also handle callbacks

var value = myFunctionSync();
myFunction(callback);
var promise = myFunction();

this approach has a tiny performance overhead (since you have to check if 
the last argument is a function to determine if you should return a promise 
or execute that function as the callback) and makes all the functions a 
little convoluted (unless you make one higher order function that you apply 
to all your callback functions to support both APIs). Furthermore async, 
higher order, overloaded functions or variable arity functions become 
impossible since you can't necessarily assume that the last argument is 
always the callback. 

(b) three function types: (1) synchronous functions; (2) async callback 
functions; and (3) async promise functions

var value = myFunctionSync();
myFunction(callback);
var promise = myFunctionDeferred();

this is ugly but explicit in terms of what to expect and permits the most 
flexibility.

(c) two function types:  (1) synchronous functions; (2) async callback 
functions;

var value = myFunctionSync();
myFunction(callback);

and promise support is left up to the user by using a nodeify() method from 
a promise library. This is my preference, but won't make my co-workers 
happy. 


With all this in mind, what's the general consensus of the NodeJS community 
on this issue? I searched google and the archives and could not find any 
blog posts or discussions that address this particular issue. What are the 
pros and cons of each approach? What if any libraries implement options (a) 
or (b)? etc.


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