TIL 2020.11.03

1. Primitive Type(원시 타입)
   • string
   • number
   • biginit
   • boolean
   • undefined
   • symboll
   • null
   • 재할당 하더라도, 처음 할당했던 값이 바뀌지 않음
2. Reference Type(참조 타입)
   • function
   • array
   • object
   • 재할당하면, 모든 것이 자동적으로 다 바꿔버림
   • save inside 'heap'

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     console.log([1,2,3] === [1,2,3]; // false  console.log({ foo: 'ba' } === { foo: 'ba' }); // false

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3. SCOPE(범위)
   • determines the accessibility of variables
   • JS receive own scope once function was created, which is lexical scope
     • check who is its outer scope
   • can put function inside function
     
     1.  Local Scope vs. Global Scope
        
        • Local Scope 
          • 'Local Variable'(지역변수) has higher priority than global variable and cannot use it outside function

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            let greeting = 'Hi'; function greet() {     let greeting = 'Hello';     console.log(greeting); // 'Hello' } console.log(greeting); // 'Hi' greet(); console.log(greeting); // 'Hi'

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            function greetSomeone() { let firstName = 'Paige'; return greeting + ' ' + firstName; } Colored by Color Scripter

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        • Global Scope
          • 'Global Variable'(전역변수) can access everywhere

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            let greeting = 'Hi'; function greetSomeone() { let firstName = 'Paige'; return greeting + ' ' + firstName; }   greetSomeone(); // => Hi Paige' firstName; // => ReferenceError

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     2. Function Scope vs. Block Scope
        
        • Block
          • starts and end with curly bracket { }
          • var vs. let
            
            • var: gets its own scope (old way)
              • connects with window object
            • let: only works inside block scope

              	let	const	var
              scope	block scope	block scope	function scope
              reassignment(재할당)	⭕️	❌	❌
              redeclaration(재선언)	❌	❌	❌

     3. Global Variable(전역변수) and Window object(윈도우객체)
        
        • will connect to function that is assigned at global scope and var varible
        • do not declare too much in global scope!
          

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          var myName = "Paige"; console.log(window.myName); // Paige   function foo() { console.log('bar'); }   console.log(foo === window.foo); // true

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     4. No Declaration(선언없이 초기화된 전역변수)
        • do not use variable without assignment
        • if it does, it becomes global variable(전역변수)
        • In order to avoid error, use 'use strict'
          

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          function showAge() { age = 24; console.log(age); } showAge(); // 24 console.log(age); // 24 age === window.age

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---

• Spread Syntax(...variable)
  
  • if spread syntax goes inside parameter, it become 'rest parameter(rest syntax)'

    function pintMaxNums(...args) { 
        console.log(args); //[10, 30, 40]
    }
    pintMaxNums(10, 30, 40);