var x; //create a variable but assign it no value
console.log("x's value is", x) //logs "x's value is undefined"

var bNoParam = new Boolean();var bZero = new Boolean(0);var bNull = new Boolean(null);var bEmptyString = new Boolean('');var bfalse = new Boolean(false);

var btrue = new Boolean(true);var btrueString = new Boolean('true');var bfalseString = new Boolean('false');var bSuLin = new Boolean('Su Lin');var bArrayProto = new Boolean([]);var bObjProto = new Boolean({});

var x = new Boolean(false);if (x) {  // this code is executed}
var x = false;if (x) {  // this code is not executed}

var x = Boolean(expression);     // use this...var x = !!(expression);          // ...or thisvar x = new Boolean(expression); // don't use this!

function sanitise(x) {  if (isNaN(x)) {    return NaN;  }  return x;}
console.log(sanitise('1'));// expected output: "1"
console.log(sanitise('NotANumber'));// expected output: NaN

> const x = 2n ** 53n;9007199254740992n> const y = x + 1n; 9007199254740993n

Number('123')  // returns the number 123Number('123') === 123  // true
Number("unicorn")  // NaNNumber(undefined)  // NaN

const theBiggestInt = 9007199254740991n
const alsoHuge = BigInt(9007199254740991)// ↪ 9007199254740991n
const hugeString = BigInt("9007199254740991")// ↪ 9007199254740991n
const hugeHex = BigInt("0x1fffffffffffff")// ↪ 9007199254740991n
const hugeBin = BigInt("0b11111111111111111111111111111111111111111111111111111")// ↪ 9007199254740991n

const string1 = "A string primitive";const string2 = String('A string primitive');

let s_prim = 'foo'let s_obj = new String(s_prim)
console.log(typeof s_prim) // Logs "string"console.log(typeof s_obj)  // Logs "object"
let s1 = '2 + 2'              // creates a string primitivelet s2 = new String('2 + 2')  // creates a String objectconsole.log(eval(s1))         // returns the number 4console.log(eval(s2))         // returns the string "2 + 2"

// Here are two symbols with the same description:let Sym1 = Symbol("Sym")let Sym2 = Symbol("Sym")
console.log(Sym1 === Sym2) // returns "false"

let sym = new Symbol()  // TypeError

let sym = Symbol('foo')typeof sym      // "symbol" let symObj = Object(sym)typeof symObj   // "object"

typeof null === 'object' // true

let o = {}let o = {a: 'foo', b: 42, c: {}}
let a = 'foo', b = 42, c = {}let o = {a: a, b: b, c: c}

let param = 'size'let config = {  [param]: 12,  ['mobile' + param.charAt(0).toUpperCase() + param.slice(1)]: 4}
console.log(config) // {size: 12, mobileSize: 4}

(function(){}).constructor === Function

var x = 10;
function createFunction1() {    var x = 20;    return new Function('return x;'); // this |x| refers global |x|}
function createFunction2() {    var x = 20;    function f() {        return x; // this |x| refers local |x| above    }    return f;}
var f1 = createFunction1();console.log(f1());          // 10var f2 = createFunction2();console.log(f2());          // 20

let today = new Date()let birthday = new Date('December 17, 1995 03:24:00')let birthday = new Date('1995-12-17T03:24:00')let birthday = new Date(1995, 11, 17)            // the month is 0-indexedlet birthday = new Date(1995, 11, 17, 3, 24, 0)let birthday = new Date(628021800000)            // passing epoch timestamp
let [month, date, year]    = ( new Date() ).toLocaleDateString().split("/")let [hour, minute, second] = ( new Date() ).toLocaleTimeString().slice(0,7).split(":")

let fruits = ['Apple', 'Banana'];
console.log(fruits.length); // 2console.log(fruits[0]);     // "Apple"
let fruits = new Array('Apple', 'Banana');
console.log(fruits.length); // 2console.log(fruits[0]);     // "Apple"

let fruits = ['Apple', 'Banana']fruits.forEach(function(item, index, array) {  console.log(item, index)})// Apple 0// Banana 1

let newLength = fruits.push('Orange')// ["Apple", "Banana", "Orange"]

let last = fruits.pop() // remove Orange (from the end)// ["Apple", "Banana"]

let first = fruits.shift() // remove Apple from the front// ["Banana"]

let newLength = fruits.unshift('Strawberry') // add to the front// ["Strawberry", "Banana"]

let removedItem = fruits.splice(pos, 1) // this is how to remove an item
// ["Strawberry", "Mango"]

let vegetables = ['Cabbage', 'Turnip', 'Radish', 'Carrot']console.log(vegetables)// ["Cabbage", "Turnip", "Radish", "Carrot"]
let pos = 1let n = 2
let removedItems = vegetables.splice(pos, n)// this is how to remove items, n defines the number of items to be removed,// starting at the index position specified by pos and progressing toward the end of array.
console.log(vegetables)// ["Cabbage", "Carrot"] (the original array is changed)
console.log(removedItems)// ["Turnip", "Radish"]

let shallowCopy = fruits.slice() // this is how to make a copy// ["Strawberry", "Mango"]

let contacts = new Map()contacts.set('Jessie', {phone: "213-555-1234", address: "123 N 1st Ave"})contacts.has('Jessie') // truecontacts.get('Hilary') // undefinedcontacts.set('Hilary', {phone: "617-555-4321", address: "321 S 2nd St"})contacts.get('Jessie') // {phone: "213-555-1234", address: "123 N 1st Ave"}contacts.delete('Raymond') // falsecontacts.delete('Jessie') // trueconsole.log(contacts.size) // 1

let myMap = new Map()myMap.set(0, 'zero')myMap.set(1, 'one')
for (let [key, value] of myMap) {  console.log(key + ' = ' + value)}// 0 = zero// 1 = one
for (let key of myMap.keys()) {  console.log(key)}// 0// 1
for (let value of myMap.values()) {  console.log(value)}// zero// one
for (let [key, value] of myMap.entries()) {  console.log(key + ' = ' + value)}// 0 = zero// 1 = one

myMap.forEach(function(value, key) {  console.log(key + ' = ' + value)})// 0 = zero// 1 = one

let mySet = new Set()
mySet.add(1)           // Set [ 1 ]mySet.add(5)           // Set [ 1, 5 ]
mySet.has(1)              // truemySet.delete(1)    // removes 1 from the set

// logs the items in the order: 1, "some text", {"a": 1, "b": 2}, {"a": 1, "b": 2} for (let item of mySet) console.log(item)