const promiseOrCallback = function(func, callback) {
  return new Promise(async (resolve, reject) => {
    try {
      const data = await func();
      if (typeof callback == "function") {
        return callback(null, data);
      }
      return resolve(data);
    } catch (err) {
      if (typeof callback == "function") {
        return callback(err, null);
      }
      return reject(err);
    }
  });
};

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