Promisify

promisify.js

// This implmentation is probably very similar to Promise.promisify--it can be helpful to visualize the details separately

var promisify = function (fn, thisArg) {
  return function () {

    var args = Array.prototype.slice.call(arguments, 0); // convert to array

    return new Promise(function (resolve, reject) {
      var callback = function () {
        var err = arguments[0];
        if (err) {
          reject(err);
        } else if (arguments.length <= 2) {
          resolve(arguments[1]);
        } else {
          var args = Array.prototype.slice.call(arguments, 0); // convert to array
          args.splice(0, 1); // remove err arg
          resolve(args);
        }
      };
      args.push(callback);
      fn.apply(thisArg, args);
    });
  };
};