promise_reduce.js

function asyncFunc(e) {
    return new Promise((resolve, reject) => {
        setTimeout(() => resolve(e), e * 1000);
    });
}

const arr = [1, 2, 3];
let final = [];

function workMyCollection(arr) {
    return arr.reduce((promise, item) => {
        return promise.then((result) => {
            console.log(`item ${item}`);
            return asyncFunc(item).then(result => final.push(result));
        })
        .catch(console.error);
    }, Promise.resolve());
}

workMyCollection(arr)
    .then(() => console.log(`FINAL RESULT is ${final}`));