ES6

1_letAndConst.js

// how come that return v in the else block will not raise error
function getx(c) {
  if (c) {
    var v = "dada"
    return v;
  } else {
    return v;
  }
}

// JS engine will will hosit var v into top of the function
function getx(c) {
  var v;
  if (c) {
    v = "dada"
    return v;
  } else {
    return v;
  }
}

// try getx(true) , getx(false)
// the problem here is v scope will be for function not only in the if block
// by using ECMAScript 6 version we can force varible into specific scope
// here it will raise error because v is only defined in if statement scope 
function getx(c) {
  if (c) {
    let v = "dada"
    return v;
  } else {
    return v;
  }
}

// correct ECMAScript 6 version
function getx(c) {
  if (c) {
    let v = "dada"
    return v;
  } else {
    return null;
  }
}

// example 1
var x = 3;
if (condition) {
  let x = 2;
  console.log(x); // print 2
}
console.log(x); // print 3

// example 2
for (var i = 0; i < 10; i++) {
    console.log(i);
}
console.log(i); // i is still accessible here

for (let i = 0; i < 10; i++) {
    console.log(i);
}
console.log(i); // throw error

// constants
// constants their values cannot be changed once initialized
// also constants like let accessible only in it's block
const x = 2;

// note const protect object from changing it's reference not value ex:
const user = {
  name: "",
  age: 0
}

// works
user.name = "Biga";
user.age = 25;

// throw error
user = {
  name: "Biga",
  age: 25
}
// so the final rule : const prevents modification of the binding, not modification of the bound value.


// let and const with GLOBAL/window scope
// GLOBAL in Node.js
// window in browser
// let and const are more safer than var by protecting your object from global/window scope
let x = 5;
const y = 10;
var z = 15;
console.log(GLOBAL.x); // print undefined
console.log(GLOBAL.y); // print undefined
console.log(GLOBAL.z); // print 15

// also let and const protect your variable from using before defination
// here remember hositing !
console.log(p); // it will print undefined
var p = 4;
// but using let or const this will raise an error
console.log(p); // raise error
let p = 5;

2_functions.js

// functions