Run promises sequentially using Array#reduce

sequential-promises.js

// https://css-tricks.com/why-using-reduce-to-sequentially-resolve-promises-works/

const sayHelloIn = (seconds) => () => new Promise((resolve, reject) => {
  setTimeout(() => {
    console.log(`Hello ${seconds} seconds later.`)
    resolve();
  }, seconds * 1000);
});

const sequentially = async (fns) => {
  return fns.reduce(async (acc, fn) => {
    await acc;
    return fn();
  }, Promise.resolve());
}

sequentially([
  sayHelloIn(1),
  sayHelloIn(2),
  sayHelloIn(1)
]).then(() => {
  console.log('done')
})