Example of the composability of promises especially when you use point-free functions.

promise.js

'use strict'

/*!
 * imports.
 */

const selectn = require('selectn')

/*!
 * mock functions.
 */

function start (val) {
  if (val) {
    return Promise.resolve('success')
  } else {
    return Promise.reject(new Error('not a success'))
  }
}

function failure () {
  throw new Error('failure')
}

function data () {
  return { status: 'done' }
}

function nope () {
  console.log('You might think this will not fire, but you would be wrong')
}

/*!
 * main.
 * Example of the composability of promises especially when you use point-free functions.
 * Uncomment one of the chains below to see it run to completion.
 */

start(1)
.then(console.log) // logs "success"
.then(data)
.then(selectn('status')) // selectn is curried so it returns a getter function with artity of 1 returning value of status key.
.then(console.log) // logs "done" --> since previous item in chain returns done, this logs it.
.then(failure)
.catch(console.error) // logs "[Error: failure]"
.then(nope) // logs "You might think this will not fire, but you would be wrong"

// start(0)
// .then(console.log)
// .then(data)
// .then(selectn('status'))
// .then(console.log)
// .then(failure)
// .catch(console.error) // logs "[Error: not a success]"
// .then(nope) // logs "You might think this will not fire, but you would be wrong"