function difficultMathCalculation(num)
{
    return new Promise((resolve, _) => setTimeout(() => resolve(num * num), 5000));
}

function bestResult(results)
{
    return Math.max(...results);
}

function doSomeStuff(array) {
    const promises = [];
    array.forEach(elem => {
        promises.push(difficultMathCalculation(elem));
    });

    return promises;
};

function getBestResult(array) {
    let promises = doSomeStuff(array);
    return new Promise((resolve, _) => {
        let allResults = [];
        Promise.allSettled(promises).then(results => {
            results.forEach(result => allResults.push(result.value));
            resolve(bestResult(allResults));
        });
    });
}

const array = [3, 2, 1, 9, 6, 5, 7, 8, 4];
getBestResult(array).then(best => console.log(best));