Async series with reduce

async-series.js

function asyncThing() {
	return new Promise(res => {
		setTimeout(() => {
			res(Math.random());
		}, 1000);
	});
}

function series(...promises) {
	return promises.reduce((p, fn) => p.then(fn), Promise.resolve());
}

series(asyncThing, asyncThing, asyncThing)
	.then(result => console.log(result));