Async-Await vs Promises in Node

async-test.js

// Before April 2017 run this with flags:
// node --harmony-async-await ./async-test.js

function sayHi (name) {
    if (name == null) throw Error('Missing name')
    return `Hi ${name}.`
}

function sayWhatsUpAsync (msg, timeoutMs) {
    console.log(`Will return in ${timeoutMs / 1000}s`)
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            if (msg == null) reject(Error('Empty message'))
            resolve(`${msg} What is up?`)
        }, timeoutMs)
    }) 
}

async function errorfree () {
    console.log('\nRunning function without errors...')
    try {
        let greeting = sayHi('Alejandro')
        greeting = await sayWhatsUpAsync(greeting, 2000)
        console.log(greeting)
    } catch (err) {
        console.error(err)
    }
}

async function errorfulSync () {
    console.log('\nRunning function with sync errors...')
    try {
        let greeting = sayHi() // errors on empty input
        greeting = await sayWhatsUpAsync(greeting, 2000)
        console.log(greeting)
    } catch (err) {
        console.error(err)
    }
}

async function errorfulAsync () {
    console.log('\nRunning function with async errors...')
    try {
        let greeting = null; // next call checks for null input and errors
        greeting = await sayWhatsUpAsync(greeting, 2000)
        console.log(greeting)
    } catch (err) {
        console.error(err)
    }
}

async function mainAsync () {
    await errorfree()    // delay next call until this one finishes
    await errorfulSync() // delay next call until this one finishes
    errorfulAsync()
}

mainAsync()

promise-test.js

// Run with:
// node promist-test.js

function sayHiAsync (name) {
  return new Promise(function (resolve, reject) {
    setTimeout(() => {
      resolve(`Hi, ${name}`)
    }, 2000)
  })
}

function sayWhatsUpAsync (msg) {
  if (msg == null) throw Error('Missing message sync')  
  return new Promise(function (resolve, reject) {
    setTimeout(() => {
      if (msg === 'Hi, JZ') reject(Error('Do not speak to JZ'))      
      resolve(`${msg} What is up?`)
    }, 2000)
  })
}

function beExcited (msg) {
  if (msg == null) throw Error('Missing message')
  return msg + '!'
}

sayHiAsync('Lily')
  .then(function (msg) {
    return msg + '1' // ES5 anonymous function style
  })
  .then((msg) => {
    return msg + '2' // ES6 anonymous function
  })
  .then(msg => msg + '3') // ES6 shortcut anonymous function
  .then(sayWhatsUpAsync) // Pass function handle directly
  .then(beExcited)
  .then(console.log) // Log final value
  .catch(err => console.error('We caught an error: ', err)) // catch error

// Intermediate sync call throws
sayHiAsync('JZ')
  .then(msg => null) // intermediate function returns null
  .then(beExcited) // this function handles null input as error
  .then(console.log) // this gets skipped
  .catch(err => console.error('We caught an error: ', err)) // catch error

// Intermediate async call throws
sayHiAsync('JZ')
  .then(sayWhatsUpAsync) // throws async error
  .then(beExcited) // this function is skipped
  .then(console.log) // this is skipped
  .catch(err => console.error('We caught an error: ', err)) // catch error