JavaScript

JavaScript JavaScript

let foo = 42; // foo 
foo = "bar"; // foo 
foo = true; // foo 

JavaScript

const foo = 42; // foo 
const result = foo + "1"; // JavaScript foo 
console.log(result); // 421

JavaScript

null typeof typeof null "object" null === null

null undefined Number toExponential() JavaScript null undefined TypeError

Null

Null null 1

Undefined

Undefined undefined 1

undefined null typeof null === "object" undefined

• return (return;) undefined
• (obj.iDontExist) undefined
• (let x;) undefined
• Array.prototype.find() Map.prototype.get() undefined

null Object.getPrototypeOf()Object.create() undefined null

null undefined undefined

(Boolean) true false 2

if...elsewhile

(Number) IEEE 754 2-1074 (Number.MIN_VALUE) 21023 (2 - 2-52) (Number.MAX_VALUE) -(253 1) (Number.MIN_SAFE_INTEGER) 253 1 (Number.MAX_SAFE_INTEGER) JavaScript Number.isSafeInteger()

• Number.MAX_VALUE Infinity
• Number.MIN_VALUE 0
• -Number.MAX_VALUE -Infinity
• -Number.MIN_VALUE -0

Infinity -Infinity Number.POSITIVE_INFINITY Number.NEGATIVE_INFINITY

10 -0 +0 0 +0 +0 === -0 true

console.log(42 / +0); // Infinity
console.log(42 / -0); // -Infinity

NaN ("Nnot a Number") JavaScript

`JavaScript 32

(BigInt) JavaScript (Number.MAX_SAFE_INTEGER)

n BigInt()

Number.MAX_SAFE_INTEGER

// 
const x = BigInt(Number.MAX_SAFE_INTEGER); // 9007199254740991n
x + 1n === x + 2n; // false9007199254740992n  9007199254740993n 

// 
Number.MAX_SAFE_INTEGER + 1 === Number.MAX_SAFE_INTEGER + 2; // true 9007199254740992

+, *, -, **, % >>>

TypeError

(String) UTF-16 16 0 1 length UTF-16 Unicode String

JavaScript

• substring()
• 2 (+) concat()

• APIfetch() Response.text()

JavaScript

(Symbol)

JavaScript callable

JavaScript

2 JavaScript Object.defineProperty() Object.getOwnPropertyDescriptor() Object.defineProperty()

value

get JavaScript

writable

enumerable

for...in

configurable

1 2 (get set)

get

undefined

set

undefined

enumerable

for...in

configurable

null [[Prototype]] [[Prototype]]

Map Map

JavaScript API 2 Date Temporal Date

:

length

Array.prototype indexOf() () push() ()

Int8ArrayFloat32Array ArrayBuffer DataView

: Map, Set, WeakMap, WeakSet

Set WeakSet Map WeakMap

Map Set < "less than" WeakMap

DOM data-* Map WeakMap

WeakMap WeakSet

: JSON

JSON (JavaScript Object Notation) JavaScript JSON JSON

JavaScript

JavaScript JavaScript

• Date() Date 1
• +
• ==

[Symbol.toPrimitive]() "default" valueOf()toString() toString() valueOf()

[Symbol.toPrimitive]() TypeError valueOf() toString() TypeError

console.log({} + []); // "[object Object]"

{} [] [Symbol.toPrimitive]() {} [] valueOf() Object.prototype.valueOf toString() {}.toString() "[object Object]" [].toString() "" "[object Object]"

[Symbol.toPrimitive]() valueOf() [Symbol.toPrimitive]() Date Symbol [Symbol.toPrimitive]() Date.prototype[Symbol.toPrimitive]() "default" "string" Symbol. prototype[Symbol.toPrimitive]()

2 Array.prototype.slice()

TypeError

NullUndefined

3

• : [Symbol.toPrimitive]("default") valueOf() toString()
• : [Symbol.toPrimitive]("number") valueOf() toString()
• : [Symbol.toPrimitive]("string") toString() valueOf()

[Symbol.toPrimitive]() valueOf toString

• JavaScript Data Structures and Algorithms (Oleksii Trekhleb)
• Computer Science in JavaScript (Nicholas C. Zakas)