Const

In ES6, the const keyword is introduced to create constant variables which can't be reassigned after their initial assignment. Attempting to reassign the const variables will get the TypeError: Assignment to constant variable.

Here is an example of using const

const PI = 3.141593

// TypeError: Assignment to constant variable
// PI = 2

We can also create the constant variable in ES5 using Object.defineProperty

Object.defineProperty(typeof global === 'object' ? global : window, "PI", {
    value: 3.141593,
    enumerable: true,
    writable: false,
    configurable: false
})

let (Block Scope Variable)

let is an alternative to var for variable declarations. Unlike var, variables declared with let have block scope(enclosed by curly braces).

let callbacks = []
for (let i = 0; i <= 5; i++)
    callbacks[i] = function() { return i * 2 }
callbacks.forEach(func => {
    console.log(func())
})
// 0
// 2
// 4
// 6
// 8
// 10

Here is the implementation with ES5

var callbacks = []
for (var i = 0; i <= 5; i++)
    (function(i){
        callbacks[i] = function() { return i * 2 }
    })(i)
callbacks.forEach(func => {
    console.log(func())
})
// 0
// 2
// 4
// 6
// 8
// 10

Arrow Functions

Arrow functions provide a more compact syntax compared to traditional function expressions, and they also behave differently in terms of how they handle the this keyword.

evens = [2, 4, 6, 8, 10]

// expression body
odds = evens.map(v => v + 1)
console.log(odds); // [3, 5, 7, 9, 11]

// statement body
threes = []
odds.map(v => {
    if (v % 3 === 0) 
        threes.push(v)
})

// lexical this
function ArrowLexicalThis() {
    this.fives = []
    this.nums = [1, 3, 5, 7, 9, 12, 15, 18, 20]
    this.nums.forEach((v) => {
        if (v % 5 === 0)
            this.fives.push(v)
    })
    console.log(this.fives)
}
new ArrowLexicalThis() // [ 5, 15, 20 ]

Here is the traditional function expressions

evens = [2, 4, 6]

odds = evens.map(function(v) { return v + 1 })

// statement body
threes = []
odds.map(function(v) {
    if (v % 3 === 0) 
        threes.push(v)
})

// lexical this
function ArrowLexicalThis() {
    this.fives = []
    this.nums = [1, 3, 5, 7, 9, 12, 15, 18, 20]
    self = this;
    this.nums.forEach(function(v) {
        if (v % 5 === 0)
            self.fives.push(v)
    })
    console.log(this.fives)
}
new ArrowLexicalThis() // [ 5, 15, 20 ]