JavaScript - Return promise AND/OR call callback?

Question

I often see in other peoples docs something like:

Callback is optional, if omitted returns a promise.

This is what I have:

export function doSomeAsync(options, callback) {

    const useCallback = (callback && typeof callback == 'function');

    const promise = new Promise((resolve, reject) => {

        // --- do async stuff here ---
        const check = (options.num === 1) ? true : false;
        setTimeout(() => {
            if (check) {
                finish(true, "Number is 1");
            } else {
                finish(false, new Error("Number is not 1"));
            }
        }, 1000);
        // ---------------------------

        function finish(ok, rtn) {
            if (useCallback) {
                if (ok) {
                    callback(null, rtn);
                } else {
                    callback(rtn, null);
                }
            } else {
                if (ok) {
                    resolve(rtn);
                } else {
                    reject(rtn);
                }
            }
        }

    });

    return (useCallback) ? false : promise;
}

The finish() function just avoids lots of if... statements scattered around.

I'm creating a promise object, whether or not I use it.

Testing like this:

doSomeAsync({ num: 1 }).then((result) => {
    console.log('p result', result);
}).catch((err) => {
    console.log('p err', err);
});

doSomeAsync({ num: 1 }, (err, result) => {
    if (err) {
        console.log('cb err', err);
    } else {
        console.log('cb result', result);
    }
});

This works, but I'm wondering if this is the best way, or if others have a better and more succinct implementation..?

What promise library are you using? Bluebird even has a dedicated function for this. — Bergi
– Bergi, Commented Apr 25, 2016 at 12:48
I'm running Node v4.2.2, with Babel for ES6 (babel-preset-es2015 6.6.0), but I haven't installed the specific babel ployfill for promises. So native Node promises I guess. — Stephen Last
– Stephen Last, Commented Apr 25, 2016 at 13:19

Bergi · Accepted Answer · 2016-04-25 12:46:58Z

24

This could be simplified if you simply always used the promise, which you're always creating anyway:

export function doSomeAsync(options, callback) {
    const promise = new Promise((resolve, reject) => {
        const check = (options.num === 1) ? true : false;
        setTimeout(() => {
            if (check) {
                resolve("Number is 1");
            } else {
                reject(new Error("Number is not 1"));
            }
        }, 1000);
    });

    if (callback && typeof callback == 'function') {
        promise.then(callback.bind(null, null), callback);
    }

    return promise;
}

Your function is always promise-based, also in the fact that it always returns a promise. The caller is simply free to ignore that. The callback argument is merely a "legacy fallback interface" (or "alternative interface" if you prefer) to using that promise.

edited Apr 25, 2016 at 12:46

Bergi

671k162 gold badges1k silver badges1.5k bronze badges

answered Apr 25, 2016 at 10:35

deceze♦

525k89 gold badges807 silver badges954 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

James Thorpe Over a year ago

Yes, this is exactly the approach I was thinking. Returning false seems a bit of a waste - also (being a .NET dev as well as JS) seems odd returning two different types from such a function.

Stephen Last Over a year ago

Thanks deceze... I've updated my question with test calls. After updating to use your code I get: p result Number is 1 and cb err Number is 1. Does the callback now only have one param?

deceze Over a year ago

@Stephen In my example above, the way the promise.then callbacks are bound, callback will receive one argument in case of success and two arguments (false and error) in case of failure. The callback is expected to have a signature of function (result, err). Design that interface whichever way you like.

Stephen Last Over a year ago

I notice the answer was updated with promise.then(callback.bind(null, null), callback) to produce the Node style callback(err, result) I was looking for. Thank you Bergi & deceze!

Adriaan Pelzer · Accepted Answer · 2019-11-14 09:01:02Z

1

You could get rid of all edge cases by always returning a promise, and define a default callback (a callback-shaped identity function) that handles the no-callback-supplied case:

const genericAsync = (stuff, callback = (e, i) => e || i) => new Promise(
  (resolve, reject) => doStuffWith(stuff, resolve, reject)
)
  .then(response => callback(null, response))
  .catch(callback);

answered Nov 14, 2019 at 9:01

Adriaan Pelzer

1,4991 gold badge9 silver badges5 bronze badges

Collectives™ on Stack Overflow

JavaScript - Return promise AND/OR call callback?

2 Answers 2

4 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

4 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related