Adds a promise property to a callback function

addPromiseToCallback.js

'use strict';

// the wrapper
function addPromiseToCallback(fn) {
  const result = function result(err, data) {
    fn(err, data);

    if (err) {
      result.reject(err);
      return;
    }

    result.resolve(data);
  };

  result.promise = new Promise((resolve, reject) => {
    result.resolve = resolve;
    result.reject = reject;
  });

  return result;
}

// calls the callback
function myLamdaFunction(cb) {
  setTimeout(() => cb(null, Date.now()), 100);
}

// calls the callback with error
function myLamdaFunctionWithError(cb) {
  setTimeout(() => cb(new Error('some error'), null), 100);
}

// my callback function
function myCallback(arg1, arg2) {
  console.log(`called back! ${arg1}, ${arg2}`);
}

const callbackWithPromise = addPromiseToCallback(myCallback);

myLamdaFunction(callbackWithPromise);
// myLamdaFunctionWithError(callbackWithPromise);

callbackWithPromise.promise
  .then((data) => {
    console.log(`Promise 1 resolved with ${data}`);
  })
  .catch(err => console.error(`Rejected with ${err}`));

I wrote this to help test an AWS lambda function. Then I realized I could have just used a promisify library to turn my lambda function into a promise. Oh well, fun anyway.