kingsubin

 

1. let vs var, hoisting

1. Variable, rw(read/write)
2. let (added in ES6)

 

• scope
  
  • global scope
  • block scope

 

Var

• var (don't ever ues this)
• var hoisting (move declaration from bottom to top)
• has no block scope
• block scope ignore

 

Constant, r (read only)

•  favor immutable data type always for a few reasons 
  • security
  • thread safety
  • reduce human mistakes

 

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2. data types

 

Variable types

• primitive, single item
  • number
  • string
  • boolean
  • null
  • undefined
  • symbol

 

• object
• box container
• function
• first-class function

 

| Note !

• immutable data types 
  • primitive types
  • frozen objects (i.e.object.freeze())

 

• mutable data types
  • all objects by default are mutable in JS

 

number

const count = 17; // integer
const size = 17.1; // decimal number
console.log(`value: ${count}, type: ${typeof count}`); // 17, number
console.log(`value: ${size}, type: ${typeof size}`); // 17.1 number

 

- number - special numeric values : infinity, -infinity, NaN

const infinity = 1 / 0;
const negativeInfinity = -1 / 0;
const nAn = 'not a number' / 2;
console.log(infinity); // Infinity
console.log(negativeInfinity); // -Infinity
console.log(nAn); // NaN

 

- bigInt (fairly new, don't use it yet)

const bigInt = 123456678012345667891234566789123456767890n; // over (-2*53) ~ 2*53
console.log(`value: ${bigInt}, type: ${typeof bigInt}`);
Number.MAX_SAFE_INTEGER;

 

 

string

const char = 'c';
const brendan = 'brendan';
const greeting = 'hello ' + brendan;
console.log(`value: ${greeting}, type: ${typeof greeting}`); // string
const helloBob = `hi ${brendan}!`; // template literals (string)
console.log(`value: ${helloBob}, type: ${typeof helloBob}`); // string

 

 

boolean

• false
  • 0
  • null
  • undefined
  • NaN
  • ""

 

• true
  • any other value

const canRead = true;
const test = 3 < 1; // false
console.log(`value: ${canRead}, type: ${typeof canRead}`); // boolean
console.log(`value: ${test }, type: ${typeof test }`); // boolean

 

 

null

let nothing = null;
console.log(`value: ${nothing}, type: ${typeof nothing}`); // null

 

 

undefined

let x;
console.log(`value: ${x}, type: ${typeof x}`); // undefined

 

 

symbol

- symbol, create unique identifiers for objects

const symbol1 = Symbol('id');
const symbol2 = Symbol('id');
console.log(symbol1 === symbol2) // false
const gSymbol1 = Symbol.for('id');
const gSymbol2 = Symbol.for('id');
console.log(gSymbol1 === gSymbol2 ) // true
console.log(`value: ${symbol1.description}`); // id

 

 

object, real-life object, data structure

const ellie = { name: 'ellie', age: 20 };
ellie.age = 21;

 

 

 

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Dynamic typing : dynamically typed language

let text = 'hello';
console.log(text.charAt(0)); // h
console.log(`value: ${text}, type: ${typeof text}`); // hello, string
text = 1;
console.log(`value: ${text}, type: ${typeof text}`); // 1, number
text = '7' + 5;
console.log(`value: ${text}, type: ${typeof text}`); // 75, string
text = '8' / '2';
console.log(`value: ${text}, type: ${typeof text}`); // 4, number

 

 

 

 

 

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※출처

www.youtube.com/channel/UC_4u-bXaba7yrRz_6x6kb_w