promises, async await

promises.js

const fs = require('fs');

// arrow functions.. These two are the same, but arrow functions are better
fs.readFile(fileName, (err, data) => {})
fs.readFile(fileName, function(err, data){})


//doing many things concurrent at the same time
async function getUsers() {
  // need this first so use await (array)
  let ids = await fetchJson("/friends/userid-123");
  // this stuff is concurrent
  let promises = ids.map((id) >= {
    return fetchJson(`/users/${id}`);
  });
  let friends = await Promises.all(promises);
  console.log(friends);
}


// regular callback-style - cannot return data, must nest callbacks
function apa(fileName) {
  fs.readFile(fileName, 'utf-8', (err, data) => {
    if (err) throw err;
    console.log(data);
  })
}


/*
2 async io functions. example using promises and async/await to avoid callback hell
*/

let readFileFrom = function(name, type) {
  return new Promise(function(resolve, reject) {
    fs.readFile(name, type, function(err, content) {
      if (err)
        reject(err)
      else
        resolve(content)
    })
  })
}

// writing the same function with arrow functions
let apa2 = (fileName) => {
    return new Promise((resolve, reject) => {
        fs.readFile(fileName, 'utf-8', (err, data) => {
            if (err) 
              reject(err)
            else
              resolve(data)
        })
    })
}

let writeData = function(name, content) {
  return new Promise(function(resolve, reject) {
    fs.writeFile(name, content, function(err) {
      if (err)
        reject(err)
      else
        resolve("ok!")
    })
  })
}

readFileFrom('package.json', 'utf-8').then(result => {
  return writeData('chaos.txt', result)
}).then(result  => {
  console.log(result)
}).catch(err => {
  console.log(err)
})

// doing the same thing but with async await
async function doFileStuff() {
  try {
    let content = await readFileFrom('package.json', 'utf-8')
    resp = await writeData('chaos.txt', content)
    console.log('file stuff done' + resp)
  } catch(err) {
    console.log(err)
  }
}
doFileStuff()