[자바스크립트] Data types, let vs var, hoisting

드림코딩 엘리 - 자바스크립트 강의 #3

 

1. Use strict

• added in ES5
• use this for Vanilla JavaScript
• ex) 'use strict';

2. Variable, rw(read/write)

• let (added in ES6)
• var (don't ever use this!)
  • var hoisting (move declaration from bottom to top) : 선언하기도 전에 할당할 수 있음
  • has no block scope : (), {} 뚫고 나옴

3. Constant, read(read only)

• use const whenever possible.
• only use let if variable needs to change.
• ex) const daysInWeeks = 7;

 

• Note!
• Immutable data types: primitive types, frozen objects (i.e. object.freeze())
• Mutable data types: all objects by default are mutable in JS
• favor immutable data type always for a few reasons:
  • security
  • thread safety
  • reduce human mistakes

 

4. Variable types

• primitive, single item (더이상 쪼갤 수 x 단위) : number, string, boolean, null, undefined, symbol
• object, box container
• function, first-class function
• data types for number
  • C : short, int, long, float, double
  • Java : byte, short, long, int, float, double
  • JS : number
    • ex) let a = 12; let b = 1.2;
  • TS : number
    • ex) let a: number = 12;
• number - special numeric values : infinity, -infinity, NaN

const infinity = 1/0;
const negativeInfinity = -1/0;
const nAn = 'not a number'/2;

• bigInt : 범위 초과하는 수 뒤에 n만 붙이면 bigInt 타입으로 인식. 크롬, 파이어폭스에서만 됨 (fairly new)
• string
• boolean
• null
• undefined
• symbol

 

5. Dynamic typing: dynamically typed language

let text = 'hello';
console.log(`value: ${text}, type: ${typeof text}`);
//⇒ value: hello, type: string

text = 1;
console.log(`value: ${text}, type: ${typeof text}`);
//⇒ value: 1, type: number

text = '7' + 5;
console.log(`value: ${text}, type: ${typeof text}`);
//⇒ value: 75, type: string

text = '8' / '2';
console.log(`value: ${text}, type: ${typeof text}`);
//⇒ value: 4, type: number