chaining async flow, like in koa2.

koa2middleware.js

// must return or await next() in koa2, otherwise promise chain will be terminated.

const delay = () => new Promise(resolve => {
    console.log('delay...')

    setTimeout(() => {
      resolve()
    }, 1000)
  })

async function asyncf (n) {
  if (n === 0) {
    return 'done'
  }

  await delay()

  return asyncf(n - 1)
}

function f (n) {
  if (n === 0) {
    return 'done'
  }

  // manually handle async
  return delay()
    .then(() => asyncf(n - 1))
}

Promise
  // f(3) => p.then(/*..*/).then(/*..*/)
  .resolve(f(3)) 
  .then(v => {console.log(v)}) // after 3s