Javascript Variables

JavaScript Variable

    JavaScript variable
    JavaScript Local variable
    JavaScript Global variable

A JavaScript variable is simply a name of storage location. There are two types of variables in JavaScript : local variable and global variable.

There are some rules while declaring a JavaScript variable (also known as identifiers).

    Name must start with a letter (a to z or A to Z), underscore( _ ), or dollar( $ ) sign.
    After first letter we can use digits (0 to 9), for example value1.
    JavaScript variables are case sensitive, for example x and X are different variables.

Correct JavaScript variables

    var x = 10; 
    var _value="sonoo"; 

Incorrect JavaScript variables

    var  123=30; 
    var *aa=320; 

Example of JavaScript variable

Let’s see a simple example of JavaScript variable.

    <script> 
    var x = 10; 
    var y = 20; 
    var z=x+y; 
    document.write(z); 
    </script> 

Output of the above example
30

JavaScript local variable

A JavaScript local variable is declared inside block or function. It is accessible within the function or block only. For example:

    <script> 
    function abc(){ 
    var x=10;//local variable 
    } 
    </script> 

Or,

    <script> 
    If(10<13){ 
    var y=20;//JavaScript local variable 
    } 
    </script> 

JavaScript global variable

A JavaScript global variable is accessible from any function. A variable i.e. declared outside the function or declared with window object is known as global variable. For example:

    <script> 
    var data=200;//gloabal variable 
    function a(){ 
    document.writeln(data); 
    } 
    function b(){ 
    document.writeln(data); 
    } 
    a();//calling JavaScript function 
    b(); 
    </script> 

JavaScript Global Variable

A JavaScript global variable is declared outside the function or declared with window object. It can be accessed from any function.

Let’s see the simple example of global variable in JavaScript.

    <script> 
    var value=50;//global variable 
    function a(){ 
    alert(value); 
    } 
    function b(){ 
    alert(value); 
    } 
    </script> 

Declaring JavaScript global variable within function

To declare JavaScript global variables inside function, you need to use window object. For example:

    window.value=90; 

Now it can be declared inside any function and can be accessed from any function. For example:

    function m(){ 
    window.value=100;//declaring global variable by window object 
    } 
    function n(){ 
    alert(window.value);//accessing global variable from other function 
    } 

Internals of global variable in JavaScript

When you declare a variable outside the function, it is added in the window object internally. You can access it through window object also. For example:

    var value=50; 
    function a(){ 
    alert(window.value);//accessing global variable  
    }