JavaScript global variable

In JavaScript, a global variable is a variable that is defined in the global scope, which means it's accessible from anywhere in your code. Global variables have a broader scope compared to variables declared within functions or other code blocks, as they can be accessed and modified across different parts of your program.

Defining Global Variables

Global variables are declared without being nested inside any function or block. They are typically declared at the top level of your script.

// Declaring global variables var globalVar1 = "Hello"; let globalVar2 = "World"; const globalVar3 = 42;

Accessing Global Variables

Global variables can be accessed and used from anywhere in your code, including within functions.

function greet() { console.log(globalVar1 + " " + globalVar2); // Output: "Hello World" } greet(); console.log(globalVar3); // Output: 42

Modifying Global Variables

Global variables can also be modified from any part of your code.

function updateGlobal() { globalVar1 = "Hi"; globalVar2 = "Universe"; globalVar3 = 99; // This will cause an error because globalVar3 is declared as a constant. } updateGlobal(); console.log(globalVar1 + " " + globalVar2); // Output: "Hi Universe"

Potential Issues with Global Variables

Using global variables excessively can lead to unintended consequences and make your code harder to maintain. For example, if multiple parts of your code modify a global variable simultaneously, it might lead to unexpected behavior or bugs. This is why it's generally recommended to use global variables sparingly and consider encapsulating data within functions or objects to control their scope and access.

function betterUpdate() { let localVar = "Local"; // A local variable within the function console.log(localVar); // Output: "Local" } betterUpdate(); // console.log(localVar); // This would result in an error because localVar is not accessible here.


Global variables are variables declared outside any function or block, accessible and modifiable throughout your entire program. However, it's a good practice to limit their usage to maintain code clarity and prevent unintended side effects.