Best es6 way to get name based results with Promise.all

Question

By default the Promise.All([]) function returns a number based index array that contains the results of each promise.

var promises = [];
promises.push(myFunc         


        

Answer 1

If you don't want to modify the format of result objects, here is a helper function that allows assigning a name to each entry to access it later.

const allNamed = (nameToPromise) => {
    const entries = Object.entries(nameToPromise);
    return Promise.all(entries.map(e => e[1]))
        .then(results => {
            const nameToResult = {};
            for (let i = 0; i < results.length; ++i) {
                const name = entries[i][0];
                nameToResult[name] = results[i];
            }
            return nameToResult;
        });
};

Usage:

var lookup = await allNamed({
    rootStatus: fetch('https://stackoverflow.com/').then(rs => rs.status),
    badRouteStatus: fetch('https://stackoverflow.com/badRoute').then(rs => rs.status),
});

var firstResult = lookup.rootStatus; // = 200
var secondResult = lookup.badRouteStatus; // = 404

---

If you are using typescript you can even specify relationship between input keys and results using keyof construct:

type ThenArg = T extends PromiseLike ? U : T;

export const allNamed = <
    T extends Record>,
    TResolved extends {[P in keyof T]: ThenArg}
>(nameToPromise: T): Promise => {
    const entries = Object.entries(nameToPromise);
    return Promise.all(entries.map(e => e[1]))
        .then(results => {
            const nameToResult: TResolved = {};
            for (let i = 0; i < results.length; ++i) {
                const name: keyof T = entries[i][0];
                nameToResult[name] = results[i];
            }
            return nameToResult;
        });
};