Variable

var / let / const

Variable is ...

The name given to the memory space where one value can be stored or refers to the memory space itself. Simply put, a variable is a named value that is a mechanism for storing and referencing values.

// Declaration + Definition
var a = 0;

// Declaration
var a;

// Definition
a = 0;

Purpose

Concept to effectively memorize data and manage it for reuse

Scope

var = Function Level Scope

let, const = Block Level Scope

Variable Declaration (Statement)

A statement that registers the variable name (identifier) ​​and notifies the JavaScript engine of the existence of the variable. When declaring a variable, use the keyword var, let, const.

• Variable declaration and assignment can express by one statement

Assignment of values.

Use the assignment operator (=) when assigning variables. The assignment operator assigns the value on the right side to the variable on the left side.

Variable Hoisting

It is a feature that acts like a declaration is pulled to the head of the code. (Applies to Var, let, const, function, class, variable, function, class)

Variable Name

Declare a variable name and free memory space to store the value. undefined is implicitly initialized by assigning a value rather than an empty space.

Initialization Phase: Get memory space to store values ​​and implicitly assign undefined.

Memories

변수에 값을 할당할 때는 이전 값 undefined가 저장되어 있던 메모리 공간을 지우고 그 메모리 공간에 할당 값 80을 새롭게 저장하는 것이 아니라 새로운 메모리 공간을 확보하고 그 메모리 공간에 할당 값 80을 저장하는 것에 주의하자.

만약 초기에 지정한 값을 새로운 값으로 변경하고, 변경 값이 저장 후 이전 값을 undefined로 교체한다. 이전 값이 더 이상 필요하지 않은 값이 된다면 가비지 컬렉터 (Garbage Collator)에 의해 자동으로 메모리에서 해제된다.

Reserved word

await / break / case / catch / class / const / continue / debugger / default / delete / do / else / enum / export / extends / false / finally / for / function / if / implements* / import / in / Instanceof / interface* / let* / new / null / package* / private* / protected* / public* / return / super / static* / Switch / this / throw / true / try / typeof / var / void / while / with / yield*