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JavaScript Scope


Scope is the set of variables you have access to.


JavaScript Scope

In JavaScript, objects and functions are also variables.

In JavaScript, scope is the set of variables, objects, and functions you have access to.

JavaScript has function scope: The scope changes inside functions.


Local JavaScript Variables

Variables declared within a JavaScript function, become LOCAL to the function.

Local variables have local scope: They can only be accessed within the function.

Example

// code here can not use carName

function myFunction() {
    var carName = "Volvo";

    // code here can use carName

}
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Since local variables are only recognized inside their functions, variables with the same name can be used in different functions.

Local variables are created when a function starts, and deleted when the function is completed.


Global JavaScript Variables

A variable declared outside a function, becomes GLOBAL.

A global variable has global scope: All scripts and functions on a web page can access it. 

Example

var carName = " Volvo";

// code here can use carName

function myFunction() {

    // code here can use carName

}
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Automatically Global

If you assign a value to a variable that has not been declared, it will automatically become a GLOBAL variable.

This code example will declare a global variable carName, even if the value is assigned inside a function.

Example

myFunction();

// code here can use carName

function myFunction() {
    carName = "Volvo";
}
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Do NOT create global variables unless you intend to.

In "Strict Mode" automatically global variables will fail.


Global Variables in HTML

With JavaScript, the global scope is the complete JavaScript environment.

In HTML, the global scope is the window object. All global variables belong to the window object.

Example

var carName = "Volvo";

// code here can use window.carName
 
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Did You Know?

Your global variables (or functions) can overwrite window variables (or functions).
Any function, including the window object, can overwrite your global variables and functions.


The Lifetime of JavaScript Variables

The lifetime of a JavaScript variable starts when it is declared.

Local variables are deleted when the function is completed.

In a web browser, global variables are deleted when you close the browser window (or tab), but remains available to new pages loaded into the same window.


Function Arguments

Function arguments (parameters) work as local variables inside functions.