Can JavaScript variables be used before declaration?
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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"
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