const promiseOrCallback = (promise, callback) => {
  if (typeof callback != "function") {
    return promise;
  }
  return promise
	.then((result) => (callback(null, result)))
  .catch(callback);
};

const foo = (a, b, callback) => {
 const promise = new Promise((resolve, reject) => {
   return resolve(a + b);                
 });
  
 return promiseOrCallback(promise, callback);
}