JavaScript Global Variables

Introduction

In JavaScript, Global variables are those variables which is declared outside of any function and it can be accessible from anywhere within the script, including these functions.

Global variables in JavaScript are essential to programming as they offer accessibility throughout a script's many sections. These are the variables that are defined outside of all functions and in the main body of the source code.

Syntax:

The syntax of global variables in JavaScript is as follows:

How to declare a Global Variable in JavaScript?

You can declare global variables in JavaScript by simply declaring them outside of any function or block scope. Global variables are declared in such a way that they are accessible from anywhere within the script.

Let's see how you declare a global variable in JavaScript:

Examples of JavaScript Global Variable

Output:

Output:

We have our global variable themeVal with the display color and have given it the value of "dark". We have two if blocks that check the value of this variable and invoke the darkTheme or lightTheme function depending on this check. The darkTheme function will therefore be called and output "current value of theme is dark" to the console.

Output:

Automatic Global Variables

When you define variables in your program without using the keywords var, let, or const, they are global variables by default. That is, they are accessible anywhere in your program.

Example

In the code below, we are declaring the variable 'a' without employing any keyword inside the function. Even though 'a' is declared inside the function, it is a global variable because we did not employ a keyword to declare it.

The output indicates that the variable 'a' is available inside and outside the function.

Output:

Benefits of a Global variable

There are some benefits of using global variables in JavaScript. Such as:

• Accessibility: In JavaScript, global variables provide a convenient way to share data across different parts of the program without passing them as function arguments.
• Ease of Use: They simplify the sharing of data between functions and modulus, which reduces the need for complex parameter passing mechanisms.
• Data Sharing: Global variables facilitate easy sharing of data between different functions or modules, eliminating the need to pass data as arguments or return values.
• Constant Values: In JavaScript, global variables are well suited for storing constant values such as pi or e, as they need to be declared only once and remain in scope until the program ends.
• Maintenance: When specific changes are required throughout the entire program, it becomes more accessible to apply via global variables.

Limitations of Global Variables

While global variables may seem convenient, they also have some limitations, such as:

• Namespace Pollution: Global variables can lead to namespace pollution, where variable names clash or conflict with each other. It can result in unexpected behaviour, bugs, or errors in your code.
• Security Risks: In JavaScript, global variables are accessible from anywhere in the code, which makes them vulnerable to security risks such as data leaks, injection attacks, or unintended modifications.
• Code Maintainability: Code that relies on global variables can be harder to maintain and debug. It becomes challenging to track the flow of data and dependencies, which leads to code that is less modular, reusable, and scalable.
• Performance Issues: In JavaScript, accessing global variables can impact the performance of your application, especially in large codebases.

Conclusion

In conclusion, the topic discussed herein, we considered the use of `global` variables in various contexts, focusing on variable naming conventions. We also considered the redeclaration of the variables' values, the scope restrictions and hoisting associated with the first `var` variable, and the `let` and `const` variables used in `JavaScript`.