js-playground.js

// 1. null vs. undefined
// null is a value, representing "nothing"
// undefined means that the variable was never assigned

const nullValue = null
const undefinedValue = undefined

let name // I declare this variable, but I don't assign it
console.log(name) // => undefined

// 2. const vs. let

// const CAN'T be reassigned
const myName = 'Gabriele' // declaration
// myName = 'Gabi' // error -> assignment to constant variable

// let CAN be reassigned
let nickname = 'Gabi'
nickname = 'Gabri'

// A constant can be changes/manipulated though!
const friends = ['Jack', 'Edo']
friends.push('Federico') // this is NOT a reassignment! I'm manipulating the object without assigning it again

// 3. Looping

// forEach is NOT modifying and giving me back another array, it's just a 'general' iteration. Inside this iteration I can do different things
// forEach always returns undefined
const myFriends = friends.forEach(friend => {
  console.log(friend.toLowerCase())
})

// map is acting on the array and giving you back a new array!
// the return of map is a new array and not undefined
const myNewFriends = friends.map(friend => friend.toLowerCase()) // one line arrow function, the ONLY case where the return is implicit
console.log(myNewFriends)

// "mapping" with a forEach (try to avoid and use reduce!)
const lowercasedFriends = []
friends.forEach(friend => lowercasedFriends.push(friend.toLowerCase()))
console.log(lowercasedFriends)

// reduce creates a new value through an iteration
// The first parameter is the new value I want to fill with the elements coming from the iteration
// The second parameter is the current element of the iteration
// After the function I pass the value of the initial element, usually 0/empty array/empty object

// Filling an empty array (instead of the forEach seen before)
const friendsLength = friends.reduce((lengths, friend) => {
  lengths.push(friend.length)
  return lengths // remember the return!
}, [])

console.log(friendsLength)

// "Filling" a number starting from 0
const totalLength = friends.reduce((length, friend) => {
  length += friend.length
  return length
}, 0)

console.log(totalLength)

// Filling an empty object
const friendsObject = friends.reduce((friendsWithLength, friend) => {
  friendsWithLength[friend.toLowerCase()] = friend.length
  return friendsWithLength
}, {})

console.log(friendsObject)