[JS] variables.js

// 1. Use strict
// added in ES 5  선언 안된 변수의 할당을 막아줌.
// use this for Vanila Javascript. 순수 바닐라 자바스크립트 작성시 되도록 써줄것.
'use strict';

// 2. Variable, rw(read/write)
// let (added in ES6)   	읽고 쓰기 가능
let globalName = 'global name';
{
  let name = 'ellie';
  console.log(name);
  name = 'hello';
  console.log(name);
  console.log(globalName);
}
console.log(name);
console.log(globalName);

// var (don't ever use this!)   절대 사용하지 말것
// var hoisting (move declaration from bottom to top)  선언만 무조건 호이스팅 (값없이 선언만 호이스탱해서 undefined)
// has no block scope		중괄호 블록스코프 무시해버림
{
  age = 4;
  var age;
}
console.log(age);

// 3. Constant, r(read only)	
// use const whenever possible.	값이 꼭 변해야하는게 아니면 무조건 const 쓰는 습관을 들일것.
// only use let if variable needs to change. 
const daysInWeek = 7;
const maxNumber = 5;

// Note! immutable 타입을 써야하는 이유!!
// Immutable data types: premitive 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: number, string, boolean, null, undefined, symbol
// object, box container
// function, first-class function

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

// number - speicla numeric values: infinity, -infinity, NaN
const infinity = 1 / 0;
const negativeInfinity = -1 / 0;
const nAn = 'not a number' / 2;
console.log(infinity);
console.log(negativeInfinity);
console.log(nAn);

// bigInt (fairly new, don't use it yet) 아직 지원안하는 브라우저 많음
const bigInt = 1234567890123456789012345678901234567890n; // over (-2**53) ~ 2*53)
console.log(`value: ${bigInt}, type: ${typeof bigInt}`);

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

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

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

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

// symbol, create unique identifiers for objects
const symbol1 = Symbol('id');
const symbol2 = Symbol('id');
console.log(symbol1 === symbol2);
const gSymbol1 = Symbol.for('id');
const gSymbol2 = Symbol.for('id');
console.log(gSymbol1 === gSymbol2); // true
console.log(`value: ${symbol1.description}, type: ${typeof symbol1}`);

// object, real-life object, data structure  나중에 자세히 설명
const ellie = { name: 'ellie', age: 20 };
ellie.age = 21;

// 5. Dynamic typing: dynamically typed language
let text = 'hello';
console.log(text.charAt(0)); //h
console.log(`value: ${text}, type: ${typeof text}`);
text = 1;
console.log(`value: ${text}, type: ${typeof text}`);
text = '7' + 5;
console.log(`value: ${text}, type: ${typeof text}`);
text = '8' / '2';
console.log(`value: ${text}, type: ${typeof text}`);
console.log(text.charAt(0));

 

 

드림코딩의 강의자료 편집본 입니다

https://www.youtube.com/watch?v=YBjufjBaxHo&list=PLv2d7VI9OotTVOL4QmPfvJWPJvkmv6h-2&index=4