async.js

const Promise = require('bluebird');

const reduceAsync = async (projection, initialValue, items) => {
    let value = initialValue;
    for (const item of items) {
        value = await projection(value, item);
    }

    return value;
}

const mapAsync = async (projection, items) => {
    return await reduceAsync(
        async (acc, item) => [...acc, await projection(item)],
        [],
        items
    );
}

const filterAsync = async (predicate, items) => {
    return await reduceAsync(
        async (acc, item) => {
            const match = await predicate(item);
            return match ? [...acc, item] : acc;
        },
        [],
        items
    );
}

const findAsync = async (predicate, items) => {
    for (const item of items) {
        const match = await predicate(item);
        if (match) {
            return item;
        }
    }

    return undefined;
}

let counter = 0;
const slowCounter = async () => {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            counter += 1;
            resolve(counter);
        }, 200);
    })
}

const slowPredicate = async (actualPredicate, item) => {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            resolve(actualPredicate(item));
        }, 200);
    })
}

const run = async () => {
    const mapped = await mapAsync(slowCounter, ["foo", "bar", "baz"]);

    console.log("Mapped:", mapped);

    const found = await findAsync(async () => await slowCounter() > 6, ["you", "can't", "find", "me!", "I'm", "hiding"]);

    console.log("Found:", found);

    const filtered = await filterAsync(
        async (item) => await slowPredicate(i => i !== "arthur", item),
        ["zaphod", "ford", "arthur", "marvin", "trillian"]);

    console.log("Filtered:", filtered);
}

run();