anti-async.js

// runtime, co se dá ukrýt

const composeAsync = (...fns) => {
  const shiftedFns = fns.slice()
  
  return (...args) => new Promise(resolve => resolve(...args))
    .then(compose.apply(this, shiftedFns.map(fn => promiseOrValue => {
      if (promiseOrValue instanceof Promise) {
        return promiseOrValue.then(fn)
      }
      return fn(promiseOrValue)
    })))
    .then(promiseResult => {
      if (promiseResult instanceof Error) {
        return Promise.reject(promiseResult)
      }

      return promiseResult
    })
      
}
// --- hezká část --- //


// "čisté" funce
// db.get() vrací promise (ale nemusí a kod vypadá stejně)
const getPost = id => db.get('posts', id)

const parsePostBody = post => post.body

const resultOr404 = result => {
  return (result ? result :
  Object.assign(new Error('Not found'), {
    code: 404,
  })
)}


const handlePostRequest = composeAsync(
  getPost,
  parsePostBody
  resultOr404
)

getPostBody(333).then(postBody => console.log(postBody))