Returning result from asynchronous function using callbacks

I have to write the body of a function getABC() which essentially has to take in responses from 3 different functions, getA()(synchronous),getB(callback)(callback driven),getC() (promise based). Ultimately I have to return a promise aggregating each function’s result, like [(result of getA),(result of getB),(result of getC)].

I am pasting the snippet of the code down below:

const [A,B,C] = ['A','B','C'];
function getA(){
 return A;
}
function getB(cb){
  setTimeout(()=>{
      cb(B);
  },10);
}
function getC()
{
   return Promise.resolve().then(()=>C)
}
function getABC()
{//Can only write the body of this function
var ARR=[];
const cb = async x =>{
const arr = [];
arr.push(getA());
arr.push(x);
arr.push(await getC());
console.log(arr); //Shows ['A','B','C'] required output
return arr;
}
let res = getB(cb);
console.log(res); //Shows undefined
return Promise.resolve().then(()=>ARR);
}
let arr = getABC().then((arr)=> console.log('Arr:',arr)); //Shows empty array ARR

` I cannot edit any other part of the code excluding the getABC() function call. I cannot wrap my head around this problem. Any help would be much appreciated. Thanks in advance!

Answer

Like so:

const[A,B,C]=["A","B","C"];function getA(){return A}function getB(t){setTimeout(()=>{t(B)},10)}function getC(){return Promise.resolve().then(()=>C)}

function getABC() {
  return new Promise(async(resolve) => {
    const a = getA()
    const c = await getC()
    getB((b) => resolve([a, b, c]))
  })
}

getABC().then(res => console.log(res))

The result from getA is returned immediately. getC returns a promise, so just await its result. Then call getB and resolve the promise in the callback.

Another option is to make getABC an async function as well. Note the difference in the way getB is called here.

const[A,B,C]=["A","B","C"];
function getA(){return A}
function getB(t){setTimeout(()=>{t(B)},10)}
function getC(){return Promise.resolve().then(()=>C)}

async function getABC() {
  const a = getA()
  const b = await new Promise(resolve => getB(resolve))
  const c = await getC()
  return [a, b, c]
}

getABC().then(res => console.log(res))

The getB line can be further simplified using a technique called eta reduction. The same technique can be used for the .then(res => console.log(res)) line.

async function getABC() {
  const a = getA()
  const b = await new Promise(getB) // eta reduced
  const c = await getC()
  return [a, b, c]
}

getABC().then(console.log) // eta reduced