Understanding Local and Global Variables in JavaScript

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.

Further Reading