Identifiers
Variables
- declare
- const (~2015)
- let (~2015)
- var
- Comparision

Identifiers

a letter(A-z, case sensitive)
a dollar sign($)
an underscore(_)
hypen is NOT allowed
NO start with numbers, NO include space
start with lower case or $ or _ , use camelCase

Variables

declear –> recall

하나의 변수(a) 값을 다른 변수(b)에 활용해도 link는 지속되지 않아 변수(a)의 값이 변해도 변수(b)는 영향 없음

declare

must identified with unique names

one statement, many variables

let name = "Mike", age = 15, gender="M"; 

declare without value
- undefined

re-declaring

can
```
var name = "Mina";
var name;
```
cannot
```
let name = "July";
let name; //error
```

using dollar sign
- concern as an alias for the main function in jQuary
using underscore
- concern as an alias for private(hidden) variables

const (~2015)

value should not be changed
type should not be changed (Arrays and Objects)
cannot be redeclared
cannot be reassigned
have block scope (cf. global scope, function scope)

does not define a constant value, defines a constant reference to a value

can change elements of constant array

can change the properties of constant object

const person = {name : "June", age : 16, height:169}
person.height = 175; //allowed
person = {name : "May", age:45, height:180}; //not allowed

let (~2015)

value can be changable
cannot be redeclared
must be declared before use

have block scope

in a block declare ONCE

let x = 10; //x=10
{
let x = 2;  //x=2
let x = 8;  //not allowed
}
      //x=10

var

changable
only if you MUST support old browsers

redeclearing problem

var x = 10; //x=10
{ 
var x = 2;  //x=2
}
          //x=2

hoisted to the top (can use the variable before it is declared)

Comparision

| | Scope | Redeclare | Reassign | Hoisted | Binds this | | —– | —– | ——— | ——– | ——- | ———- | | var | N | Y | Y | Y | Y | | let | Y | N | Y | N | N | | const | Y | N | N | N | N |