I have a number of functions that are written to accept two callbacks and some parameters that I would like to Promisify. Example:

function myFunction(successCallback, failureCallback, someParam)

Given the above function how would I Promisify both the successCallback and failureCallback using a Promise library such as Bluebird?

I have tried this but it returns undefined:

const myFunctionAsync = Promise.promisify(myFunction);
console.log(await myFunctionAsync('someParam')); // undefined

A working but overly verbose solution:

const myFunctionAsync = new Promise((resolve, reject) => 
    myFunction(success => resolve(success), failure => reject(failure))
);
console.log(await myFunctionAsync('someParam')); // success

I'm looking for a way to convert these awkward multiple callback functions into Promises without wrapping each one.

Many thanks.

I have a number of functions that are written to accept two callbacks and some parameters that I would like to Promisify. Example:

function myFunction(successCallback, failureCallback, someParam)

Given the above function how would I Promisify both the successCallback and failureCallback using a Promise library such as Bluebird?

I have tried this but it returns undefined:

const myFunctionAsync = Promise.promisify(myFunction);
console.log(await myFunctionAsync('someParam')); // undefined

A working but overly verbose solution:

const myFunctionAsync = new Promise((resolve, reject) => 
    myFunction(success => resolve(success), failure => reject(failure))
);
console.log(await myFunctionAsync('someParam')); // success

I'm looking for a way to convert these awkward multiple callback functions into Promises without wrapping each one.

Many thanks.

• javascript
• ecmascript-6
• bluebird
• es6-promise

Share Improve this question Follow asked Oct 12, 2017 at 19:31 RobulaRobula 71611 gold badge1313 silver badges2929 bronze badges 2

• Well you will need to create (wrap) your callbacks in a promise. Whether you do it yourself or a library has a handy utility method it seems to be teh same thing. – Chris Phillips Commented Oct 12, 2017 at 19:38
• True that, maybe I am getting too hung up on the Promisify function which works great for Node style functions where the last parameter is a callback. – Robula Commented Oct 12, 2017 at 19:41

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2 Answers 2

Sorted by: Reset to default 3

You could write your own version of a promisify function, that would take that function signature into account:

function myFunction(successCallback, failureCallback, someParam) {
    setTimeout(_ => successCallback('ok:' + someParam), 100);
}

function myPromisify(f) {
    return function(...args) {
        return new Promise( (resolve, reject) => f(resolve, reject, ...args) );
    }
}

async function test() {
    const myFunctionAsync = myPromisify(myFunction);
    console.log(await myFunctionAsync('someParam')); // success
}

test();

Bluebird or otherwise, it's not that hard to promisify functions. Your solution is overly verbose. Try:

const myFunctionAsync = (...a) => new Promise((r, e) => myFunction(r, e, ...a));

Yeah, this is wrapping each one, but with one line per function, unless your functions all follow some pattern and you have them in array, it's not that big a deal. I.e. you're assuming additional args are at the end rather than the beginning.