Promises best practice: Promise.resolve(false) or Promise.reject()?

Question

I have a function exists that checks somethings and returns a promise, and a function isAuthorized that calls it. I can use it via two ways:

// Way A: resolve() or reject()

export const existsA = (id) => {
    return getSomethingById(id)
        .then((result) => {
            if (!result ) {
                return Promise.reject(new Error(''));
            }

            return Promise.resolve();
        })
        .catch(error => Promise.reject(new Error(error)));
};

exports.isAuthorizedA = (req, res, next) => {
    existsA(req.user.id)
        .then(next)
        .catch(next(Boom.forbidden()));
};

// Way B: resolve(true) or resolve(false)

export const existsB = (id) => {
    return getSomethingById(id)
        .then((result) => {
            if (!result ) {
                return Promise.resolve(false);
            }

            return Promise.resolve(true);
        })
        .catch(error => Promise.reject(new Error(error)));
};

exports.isAuthorizedB = (req, res, next) => {
    existsB(req.user.id)
        .then((result) => (result ? next() : next(Boom.forbidden())))
        .catch(next(Boom.forbidden()));
};

Which way is correct ? Thank you by advance.

Answer 1

Best practice is to reject the promise of any error occurrence(uncaught exceptions) so better would be

const existsA = (id) => getSomethingById(id);

const isAuthorizedA = (req, res, next) => {
    existsA(req.user.id)
        .then(next)
        .catch(next(Boom.forbidden()));
};

handle all your fail case in getSomethingById() and reject the errors from there. If you want null or any falsy value as error then write

getSomethingById(id) => {
  new Promise(function(resolve, reject) {
  // your operations    
  if(!value)
    reject(new Error('your code'))
  else
    resolve(123)
});

as for your use case you don't want a false as a result, I mean sometimes in your promise you expect a false as a positive result like following

checkIfPostIsLiked(userId, postId)
.then(isLiked => {
    if (isLiked) return unLikePost(userId, postId)
    return likePost(userId, postId)
})
.catch(next(Boom.forbidden()));

If you are considering false as an error then this might not work.

So as a best practice whenever you want to throw an error or mark that case as a fail case then always reject promise and handle it in the catch block.