Concept of Promise in Javascript

app.js

const fs = require('fs');

(() => {
  return new Promise(async (resolve, reject) => {
    try {
      let file = fs.readFileSync(`${__dirname}/app.js`)
      resolve(file)
    } catch (err) {
      reject(`Oops! \n ${err}`)
    }
  })
})()
.then(file => console.log(file))
.catch(err => console.log(err))


const promiseOne = new Promise((resolve, reject) => {
  try {
    let file = fs.readFileSync(`${__dirname}/app.js`)
    resolve(file)
  } catch (err) {
    reject(`Oops! \n ${err}`)
  }
})

const promiseTwo = Promise.resolve('crazyoptimist')

Promise.all([promiseOne, promiseTwo])
.then(values => values.map((val, key) => console.log(`Promise No ${key + 1} Output: \n${val}`)))
.catch(error => console.log(error))
.finally(() => console.log('Finally, operation finished'))

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