Tutorials
THE WORLD'S LARGEST WEB DEVELOPER SITE

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

}
Try it Yourself

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

}
Try it Yourself

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";
}
Try it Yourself

Do NOT create global variables unless you intend to.

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
 
Try it Yourself

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.

Global variables are deleted when you close the page.

Function Arguments

 

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