promise-and-callback-support

example-one.js

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);
}

example-two.js

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);
}