ES6 tl;dr; for beginners

gistfile1.js

// ES6 tl;dr; for beginners
 
// 1. variables
 
// `const` & `let` are scoped at the block level
if(true) {
  let foo = "bar"
}
foo // ReferenceError
 
// `const` prevents reassignation
const foo = "bar"
foo = "baz" // Errors
 
// 2. functions
 
// can be that simple
const simpleSquare = (number) => Math.pow(number, 2)
 
// can be more complex when body is wrapped between curly braces
const square = (number) => {
  if(isNaN(number)) {
    throw new TypeError()
  }
  return Math.pow(number, 2)
}
// NOTE: to return an object, you have to wrap it between parentheses,
// otherwise it would think it's the function body:
const getEmptyObject = () => ({})
 
// fat-arrow functions have no .prototype,
// & their thisValue is the one of the upper-scope
 
const object = {
  // this is a new syntax that replaces `isActiveItem: function isActiveItem(item) {`
  isActiveItem(item) {
    return item.active
  },
  getActiveItems() {
    return array.filter((item) => {
      // `thisValue` the one from the `getActiveItems` body
      return this.isActiveItem(item)
    })
  }
}
 
// 3. classes
 
// basically just sugar for prototypal inheritance
class User {
  constructor(name) {
    this.name = name
  } // no commas here
  getName() {
    return this.name
  }
}
 
let user = new User("foo")
user.getName() // "foo"
 
// 4. maps & sets
 
// basically they're like object, but accepting anything as a key
 
const map = new Map()
map.set(user, "ok")
map.get(user) // "ok"
 
const set = new Set()
set.add(user)
for(let item of set) {
  console.log(item)
}
// logs `user`
set.delete(user)
 
const weakMap = new WeakMap()
map.set(user, "ok")
map.get(user) // "ok"
user = null
// weakMap doesn't keep `user`
 
// 5. modules
 
import Module from "Module" // like `const Module = require("Module")`
 
export default 1 // like `module.exports = 1`
 
export function bar() {} // like `exports.foo = function foo(){}`
 
// 6. destructuring
 
const options = {
  onSuccess: () => {},
  onFailure: () => {},
}
 
const {onSuccess, onFailure} = options
 
onSuccess // () => {}
 
// 7. objects enhancements
 
const enhancedObject = {
  onSuccess, // like `onSuccess: onSuccess`,
  [1 + 1]: "foo", // like `object[1 + 1] = "foo"`
}
 
// 8. template strings
 
const unread = 1
const value = `you have ${ unread } message${ unread !== 1 ? `s` : `` }`
 
value // you have 1 message
 
// 9. function arguments
 
const noop = () => {}
 
function after(duration = 0) { // default values
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve()
    }, duration)
  })
}
 
after() // will wait 0ms
after(100) // will wait 100ms

function toArray(...args) {
 // rest param, like if you did `const args = Array.prototype.slice.call(arguments, index)`
  return args
}

function sum(...args) {
  return args.reduce((acc, item) => acc + item, 0)
}

sum(...[1,2,3]) // like `sum.apply(undefined, [1, 2, 3])`