stay JavaScript Variables can be defined using keywords var,let or const.
We can understand var
,let
and const
Before different , We need to know a JavaScript
The concept of Scope
.
Scope
Essentially, it refers to where these variables are available .
Global scope
Globally declared variables have global scope outside any function .
Global variables can be from JavaScript
Access anywhere in the program .
Local scope
Variables declared within a function have function scope .
Local variables can only be accessed from within the function that declares them .
Block scope
The code block is JavaScript
The code between curly braces . In block {}
Variables declared within have block scope .
Use it carefully var
A variable declared by a keyword cannot have a block scope .
var
var
Declarations can be global or local ( In the function ).
When var
When a variable is declared outside a function , Scope is global . var
When declared in a function, it is the function scope .
use var
Keyword declared variables can be redeclared like this
Or their values can be updated like this
let
let
Now it's the first choice for variable declarations . It's no surprise , Because it's right var
Declared improvements . It also solves var
Some remaining problems , Let's see how it works .
let
Is block wide , So the variables declared in the block let
Can only be used in this block .
use let
The variable declared by keyword cannot be redeclared. It will throw such an error
let Variables can be updated within their scope like this
Constant
Use const
Variables that hold constant value declarations .const
Declaration and let
The statement has some similarities . And let
The statement is the same ,const
Declarations can only be accessed within the block in which they are declared .
But with const
Declared variables can neither be redeclared nor reassigned
Last
I hope this article can help you understand var
、let
and const
The difference between , Thank you for your patience , If articles and notes can give you a little help or inspiration , Please don't be stingy with your Star and Fork, Articles are updated continuously , Your affirmation is my biggest driving force