Sequential execution of promises

promise_seq.js

const a = (n, timeout) =>
  () =>
    new Promise((resolve, reject) => {
      setTimeout(
        () => {
          console.log(n)
          resolve(n)
        },
        timeout)
    })

const promises = [
  a(1, 300),
  a(2, 200),
  a(3, 100)
]

promises.reduce(
  (prev, current) => prev.then(current),
  Promise.resolve())

output

1
2
3