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Understanding Local and Global Variables in JavaScript

  Author : Shailendra Chauhan
Updated On : 24 Sep 2013
Total Views : 17,439   
 

Like other programming languages, JavaScript also has local and global variables. But the declaration and scope of a variable in JavaScript is different from others programming languages. In this article, I am going to explain the difference between local and global variables in JavaScript.

Types of Variables in JavaScript

  1. JavaScript Local Variable

    A variable that is declared inside a function definition is called local and has scope to that function only. JavaScript does not support block scope in which a set of curly braces {. . .} defines a new scope.

    <script>
    // Global variable.
    var a = "Dot Net Tricks !";
    function Show() {
     // A local variable is declared in this function.
     var a = "Hello World !";
    
     alert("Value of 'a' inside the function " + a); //Hello World !
    }
    
    alert("Value of 'a' outside the function : " + a); //Dot Net Tricks !
    </script>
    
  2. JavaScript Global Variable

    A variable that is declared outside of a function definition is called a global variable and its scope is throughout your program means its value is accessible and modifiable throughout your program.

    <script >
    // Global variable.
    var a = "Dot Net Tricks !";
    function Show() {
     // A Local variable is declared in this function.
     var a = "Hello World !";
     alert("Value of 'a' inside the function :" + a); //Hello World !
    
     //b will have global scope
     b = "Hello JavaScript !";
     Display();
    }
    
    alert("Value of 'a' outside the function : " + a); //Dot Net Tricks !
    
    function Display() {
     //Since b variable has global scope
     alert("Value of 'b' outside the function : " + b); //Hello JavaScript !
    }
    </script>
    

Note

  1. A variable that is declared inside a function using the var keyword, will have a local scope.

  2. A variable that is declared inside a function without var keyword, will have a global scope means acts like a global variable.

What do you think?

I hope you will enjoy the tips while programming with JavaScript. I would like to have feedback from my blog readers. Your valuable feedback, question, or comments about this article are always welcome.


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