JS 기초 강의(ES5+) - # Data type (2)

1. "Variable" - 변수, rw(read/write)

• 변수 선언 - "let"
• "var' 사용 금지 - hoisting, scope 문제

  let name = 'minsoo';

2. "Constant" - 변하지 않는 데이터 타입, r(read only)

• 'let'은 값을 언제든지 변경 가능하기 때문에 'const' 사용

  const daysInWeek = 7;

3. "Variable types"

• number

  const count = 17; // integer
  const size = 17.1; // decimal number

• "typeof" - 변수 타입 확인하기

  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;

• bigInt - {(-2^53) ~ 2^53 }을 초과하는 범위

  const bigInt = 1234567890123456789012345678901234567890n;

• string

  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

  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

  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

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

• 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));