An example of a function that returns a promise and takes a callback

both-ways.js

var when = require("when");

function makePromiseCallback(label, callback) {
  var deferred = when.defer();

  function actualCallback(err, data) {
    if (err)
      console.log(label || "some label", {err: err, data: data});
    if (err)
      deferred.reject(err);
    else
      deferred.resolve(data);
    callback && callback(err, data);

  }

  return {promise: deferred.promise, callback: actualCallback};

}

function longRunningOperation(n, callback) {

  var pc = makePromiseCallback("longRunningOperation", callback);

  setTimeout(function () { pc.callback(undefined, n * n)}, 1000 );

  return pc.promise;

}


exports.longRunningOperation = longRunningOperation;