codemilli

if (true) {
  console.log('a => ', a); // undefined
  console.log('b => ', b); // b is not defined

  var a = 'hacker';
  let b = 'pinky';
}

codemilli

var a = 'hacker'; // a = 'hacker'
var a = 'moon';   // a = 'moon'

let b = 'hacker'; // b = 'hacker'
let b = 'pinky';  // Identifier 'b' has already been declared

codemilli

const a = 'haha'; // a = 'haha'
a = 'hihi';       // Assignment to constant variable

codemilli

const a = { prop: 'haha' };
console.log('prop => ', a.prop); // haha

a.prop = 'hihi';
console.log('prop => ', a.prop); // hihi

codemilli

var s = new Set();
s.add(1).add(2).add(1);

console.log('Size of s => ', s.size); // 2
console.log('s has 2 => ', s.has(2)); // true

s.delete(2);
console.log('s has 2 => ', s.has(2)); // false

s.clear();

codemilli

let ws = new WeakSet();
let value = {a: 'hacker'};
ws.add(value).add({b: 'moon'});

console.log(ws); // WeakSet {Object {a: "hacker"}, Object {b: "moon"}}
// after the garbage collector has run
console.log(ws); // WeakSet {Object {a: "hacker"}}

codemilli

let map = new Map();
let obj = {a: 'haha'};

map.set(obj, 'hihi').set(1, 1).set(1, 2);

console.log('map has 1 => ', map.has(1));  // true
console.log('value of 1 => ', map.get(1)); // 2

map.delete(1);
console.log('map has 1 => ', map.has(1));  // false

codemilli

let wm = new WeakMap();
let key = {a: 1};
wm.set(key, 1).set({b: 2}, 2);

console.log(wm); // WeakMap {Object {a: 1} => 1, Object {b: 2} => 2}
// after the garbage collector has run
console.log(wm); // WeakMap {Object {a: 1} => 1}

codemilli

let a = (a) => {
    return a + 1;
};

let b = b => b + 1;
let c = () => ({hacker: 'moon'});

codemilli

let obj1 = {
    cb: function () {
        let that = this;
        let fun = function () {
            console.log('this => ', this); // undefined
            console.log('that => ', that); // { cb: [Function] }
        };

        fun();
    }