How to write code that supports BOTH callbacks and promises

test2.js

var Q = require('q');

/**
 * Promise functions that take both node callbacks and promises
 */

function half(cb) {
  var d = Q.defer();
  setImmediate(function() {
    d.resolve(21);
  })
  return d.promise.nodeify(cb);
}

function doubleIt(answer, cb) {
  var d = Q.defer();
  setImmediate(function() {
    d.resolve(answer*2);
  })
  return d.promise.nodeify(cb);
}

function solve(cb) {
  return half()
    .then(doubleIt)
    .nodeify(cb);
}

/**
 * Use the node callback interface to solve()
 */

solve(function (err, result) {
  if (err) throw err;
  console.log('nodeback', result);
});

/**
 * Use the promise interface to solve()
 */
solve().then(function (answer) {
  console.log('promise', answer);
});