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();