index.js

/**
 * Асинхронный reduce
 * @param {any[]} input
 * @param {Function} iterator
 * @param {any} initialValue
 * @return {Promise}
 */
function asyncReduce(input, iterator, initialValue) {
    let result = Promise.resolve(initialValue);
    input.forEach((promise) => {
        result = result.then((prevValue) =>
            promise().then((value) =>
                iterator(prevValue, value)
            )
        )
    });
    return result
}

let a = () => Promise.resolve('a');
let b = () => Promise.resolve('b');
let c = () => new Promise(resolve => setTimeout(() => resolve('c'), 100));
asyncReduce(
    [a, c, b],
    (acc, value) => [...acc, value],
    ['d']
).then(results => {
    console.log(results); // ['d', 'a', 'c', 'b'];
});