Scope

JavaScript has the following kinds of scopes:

Global scope: The default scope for all code running in script mode.
Module scope: The scope for code running in module mode.
Function scope: The scope created with a function .

In addition, variables declared with let or const can belong to an additional scope:

Block scope: The scope created with a pair of curly braces (a block ).

A function creates a scope, so that (for example) a variable defined exclusively within the function cannot be accessed from outside the function or within other functions. For instance, the following is invalid:

                                    
                                        function
                                        exampleFunction
                                        (
                                        )
                                        {
                                        const
                                        x =
                                        "declared inside function"
                                        ;
                                        // x can only be used in exampleFunction
                                        console.
                                        log
                                        (
                                        "Inside function"
                                        )
                                        ;
                                        console.
                                        log
                                        (
                                        x)
                                        ;
                                        }
                                        console.
                                        log
                                        (
                                        x)
                                        ;
                                        // Causes error

However, the following code is valid due to the variable being declared outside the function, making it global:

                                    
                                        const
                                        x =
                                        "declared outside function"
                                        ;
                                        exampleFunction
                                        (
                                        )
                                        ;
                                        function
                                        exampleFunction
                                        (
                                        )
                                        {
                                        console.
                                        log
                                        (
                                        "Inside function"
                                        )
                                        ;
                                        console.
                                        log
                                        (
                                        x)
                                        ;
                                        }
                                        console.
                                        log
                                        (
                                        "Outside function"
                                        )
                                        ;
                                        console.
                                        log
                                        (
                                        x)
                                        ;

Blocks only scope let and const declarations, but not var declarations.

                                    
                                        {
                                        const
                                        x =
                                        1
                                        ;
                                        }
                                        console.
                                        log
                                        (
                                        x)
                                        ;
                                        // ReferenceError: x is not defined

Scope

See also