run asynchronous tasks sequentially

asynchronous_tasks_sequentially.js

/**
 * Returns promise which runs given callback functions in series and
 * collect results in array.
 *
 * @param {array} promises - array of callback functions
 */
function promiseAllSeq(promises) {
    var data = [];
    return promises.reduce((p, fn) => p.then(fn).then(i => (data.push(i), data)), Promise.resolve());
}

const p = [
    () => new Promise(resolve => setTimeout(() => {
        return resolve('Hello')
    }, 100)),
    () => Promise.resolve('world'),
    () => new Promise(resolve => setTimeout(() => {
        return resolve('!')
    }, 50))
];

promiseAllSeq(p).then(results => console.log(results)); // [ 'Hello', 'world', '!' ]