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Everything you need to know about VaR, let and const in JavaScript

2021-08-26 14:16:27 wscats

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 .

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use var Keyword declared variables can be redeclared like this

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Or their values can be updated like this

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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 .

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use let The variable declared by keyword cannot be redeclared. It will throw such an error

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let Variables can be updated within their scope like this

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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 .

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But with const Declared variables can neither be redeclared nor reassigned

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Last

I hope this article can help you understand varlet 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

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