Expose API with Promises and Optional Callback Feature

The API pattern below allows users to use it as a promise or as a callback-based API (if an optional callback argument is passed in):

function asyncFn(arg1, arg2, callback) {
  return new Promise(function (resolve, reject) {
    // When error occurs
    if (someError) {
      const err = new Error('Invalid input');
      if (callback) {
        callback(err);
      }
      return reject(err);
    }

    // No error
    if (callback) {
      callback(null, result);
    }
    resolve(result);
  });
}

Some notes:

• The async API checks presence of callback function before calling it
• A promise is always created and completed regardless if users opt for the callback option.