Promises and callback

domenic.js

function add (arg1, arg2, cb) {
  return (new Promise(reject, resolve) {
  
 
    if (typeof arg1 !== 'number') {
      reject(new TypeError('arg1 should be a number'));
    }
 
    if (typeof arg2 !== 'number') {
      reject(new TypeError('arg2 should be a number'));
    }
 
    resolve(arg1 + arg2);
  }).then(function (result){ cb(null, result);}, cb);
}

gist.js

function add (arg1, arg2, cb) {
  return new Promise(reject, resolve) {
    if (cb) {
      reject = function (err) {
        cb(err)
      };
      resolve = function (result) {
        cb(null, result);
      }
    }

    if (typeof arg1 !== 'number') {
      reject(new TypeError('arg1 should be a number'));
    }

    if (typeof arg2 !== 'number') {
      reject(new TypeError('arg2 should be a number'));
    }

    resolve(arg1 + arg2);
  }
}