Async function handling examples

async.js

// https://developer.mozilla.org/en-US/docs/Learn/JavaScript/Asynchronous/Introducing
var inputString = 'Hello World';

function callbackStyleFunction (input, cb) {
  //if (someErrorCondition) return cb('something bad happened');
  cb(null, input);
};

const promiseStyleFunction = function (input) {
  return new Promise(function (resolve, reject) {
    //if (someErrorCondition) return reject('something bad happened')
    resolve(input)
  })
}

// Example 1 - callback
callbackStyleFunction(inputString, function (errorFromCallback, resultOfCallback) {
  console.log('callback finished');
  console.log(errorFromCallback);
  console.log(resultOfCallback+' from callback');

  // Example 2 - chained promises (this will fire pretty much at the same time as example 3)
  const { promisify } = require('util')
  const returnInput = promisify(callbackStyleFunction)
  returnInput(inputString)
  .then(function (resultOfCallbackAsPromise) {
    console.log('do something else after')
    console.log(resultOfCallbackAsPromise+' from 1st chained function')
    return resultOfCallbackAsPromise
  })
  .then(async function (resultOfCallbackAsPromise) {
    console.log('finally do something here')
    console.log(resultOfCallbackAsPromise+' from 2nd chained function')
    const resultOfPromise = await promiseStyleFunction(inputString)
    console.log(resultOfPromise+' from promise awaited in the async function that is the 2nd chained function')
  })
  .catch(function (errorFromCallbackAsPromise) {
    console.log('whoops, chain of promises broke')
    console.log(errorFromCallbackAsPromise)
  })

  // Example 3 - async / await the callback function (this will fire pretty much the same time as example 2, just after it in reality)
  ;/*here be dragons*/(async function () {
    try {
      var resultOfCallbackAsPromise = await returnInput(inputString)
      console.log('a bit easier')
      console.log(resultOfCallbackAsPromise+' from callback converted to promise and awaited in an async IIFE')
      const resultOfPromise = await promiseStyleFunction(inputString)
      console.log(resultOfPromise+' from promise awaited in an async IIFE')
    } catch (errorFromCallbackAsPromise) {
      console.log('whoops, async did not await the callback function')
      console.log(errorFromCallbackAsPromise)
    }
  })()
})

// Example 4 - async / await the promise function (this will fire pretty much the same time as example 1, just after it in reality)
async function showTheInput () {
  try {
    const resultOfPromise = await promiseStyleFunction(inputString)
    console.log('a bit more easier')
    console.log(resultOfPromise+' from promise awaited in an async function')
  } catch (errorFromPromise) {
    console.log('whoops, async did not await the promise function')
    console.log(errorFromPromise)
  }
}
showTheInput()