How to Promisify a function that has multiple callback parameters?

Question

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.

Answer 1

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(); 

Answer 2

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. Ie you're assuming additional args are at the end rather than the beginning.