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This article mainly introduces the JavaScript variable can be used before the declaration of the relevant knowledge, the content is detailed and easy to understand, the operation is simple and fast, with a certain reference value, I believe that after reading this JavaScript variable can be used before the declaration of the article will have a harvest, let's take a look at it.

The JavaScript var variable is allowed to be redeclared anywhere in the program:

Example

Var x = 2; / / allow

Var x = 3; / / allow

X = 4; / / allow

It is not allowed to redeclare or reassign existing var or let variables to const in the same scope or block:

Example

Var x = 2; / / allow

Const x = 2; / / not allowed

{

Let x = 2; / / allow

Const x = 2; / / not allowed

}

It is not allowed to redeclare or assign an existing const variable in the same scope or block:

Example

Const x = 2; / / allow

Const x = 3; / / not allowed

X = 3; / / not allowed

Var x = 3; / / not allowed

Let x = 3; / / not allowed

{

Const x = 2; / / allow

Const x = 3; / / not allowed

X = 3; / / not allowed

Var x = 3; / / not allowed

Let x = 3; / / not allowed

}

It is allowed to redeclare const in another scope or block:

Example

Const x = 2; / / allow

{

Const x = 3; / / allow

}

{

Const x = 4; / / allow

}

Promote

Variables defined by var are promoted to the top. If you do not know what Hoisting is, please learn this chapter.

You can use it before declaring the var variable:

Example

CarName = "Volvo"; / / you can use carName here

Var carName

Variables defined by const are not promoted to the top.

The const variable cannot be used before declaration:

Example

CarName = "Volvo"; / / you cannot use carName here

Const carName = "Volvo"

This is the end of the article on "can JavaScript variables be used before declaration?" Thank you for reading! I believe you all have a certain understanding of the knowledge of "can JavaScript variables be used before declaration". If you want to learn more, you are welcome to follow the industry information channel.

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