Understanding Local and Global Variables in JavaScript

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06 Sep 2022

Intermediate

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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

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>

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

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