JavaScript Global Variables

Last Updated : 12 Dec, 2024

Global variables in JavaScript are variables that can be accessed and modified from anywhere in the code, regardless of the scope where they were declared. These variables are stored in the global object (window in browsers and global in Node.js).

The declaration of global variables and accessing them.

JavaScript

// Global variable
let courseName = 'GeeksforGeeks';

function myFunction() {
    console.log(courseName);
}

myFunction()

Output

GeeksforGeeks

Example: We will declare a global variable inside a function and access it outside that function.

JavaScript

function myFunction() {

    // Considered global
    courseName = 'GeeksforGeeks';
    console.log( courseName );
}

myFunction();

console.log( courseName );

Output

GeeksforGeeks
GeeksforGeeks

Interesting facts about Global Objects

Global Variables Are Part of the Global Object

JavaScript

var myVar = "Hello";
console.log(window.myVar);

Undeclared Variables Become Global (in Non-Strict Mode)

JavaScript

function createGlobal() {
  myGlobal = "I'm global!";
}
createGlobal();
console.log(myGlobal);

Output

I'm global!

Global Variables Can Be Overwritten

JavaScript

var appName = "MyApp";

// Another script overwrites it
var appName = "OtherApp";

console.log(appName);

Output

OtherApp

Strict Mode Restricts Globals

JavaScript

"use strict";
myGlobal = "Oops"; // ReferenceError: myGlobal is not defined

Advantages:

Convenience: Easily share data across different parts of the application.
Persistent Data: Retains value throughout the program’s execution.

Limitations of Global Variables

Naming Conflicts: Variables with the same name can unintentionally overwrite each other.
Debugging Challenges: It’s harder to trace changes to global variables.
Memory Usage: Excessive use can lead to memory leaks, as global variables persist throughout the application’s lifecycle.