convert a callback-style node method to promise

example.js

var User = {
  getData: function(userId, cb){
    AsyncDB.query("GET DATA FOR userId", function(err, data){
      cb(err, data);
    });
  }
};

promisify(User, 'getData');// This will append method called `getDataAsync`

User.getDataAsync('23ss23').then(function(data){
  console.log('User data is ' + data);
}).fail(function(err){
  console.error(err);
});

promisify.js

var promisify = function (API, methodName) {
	API[methodName + 'Async'] = function () {
		var gen = {};
		var promise$ = new Promise(function (resolve, reject) {
			gen.resolve = resolve;
			gen.reject = reject;
		});
		var allArgs = Array.prototype.slice.call(arguments);
		allArgs.push(function (err, data) {
			if (err) {
				gen.reject(err);
				return;
			}
			gen.resolve(data);
		});
		API[methodName].apply(API, allArgs);
		return promise$;
	};
};