promisfy.js

var promisfy = function (fn, ctx) {
  return function () {
    var args = arguments;
    
    return new Promise(function (resolve, reject) {
      var cb = function () {
        var err = Array.prototype.shift.call(arguments);
        if (err) {
          reject(err);
        } else {
          resolve.apply(null, arguments);
        }
      };

      Array.prototype.push.call(args, cb);

      fn.apply(ctx || null, args);
    });
  };
};