What is the difference between global and local variables in JavaScript?

There are different types of variables present in JavaScript such as global and local. In this article, we are going to see some common differences between them with examples, starting with an intro to the end. Stick with this article and you will understand everything about the global and local variables of JavaScript.

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Introduction

Brief overview of the topic

The global and local variables in JavaScript are a bit easy and confusing at the same time. Once you got a clear idea of how to work with them, you will never gonna get confused while selecting a global or local variable. It all depends on the situation in which you want to declare a variable. This article will cover everything about it and I will also tell you when you should use global or local variables.

Explanation of the importance of understanding the difference between global and local variables in JavaScript

The importance of understanding the difference between global and local variables in JavaScript is really important. Because you as a programmer have to decide where your variables can be accessible and where not, there comes the local and global variables into play.

To put it into simple words the global variables can be used anywhere in the program, on the other hand, the local variables are only accessible in the block where they are declared such as if-else, function, for-while loops, etc.

Definition of Global Variables

Explanation of what a global variable is

The global variable in JavaScript is the variable that can be accessed from anywhere in our program. Which means they don’t have any limits compared to the local variables. Or the variables which are declared outside any blocks such as if-else, for-while loops, functions, etc known as global variables.

How global variables are defined and accessed in JavaScript

A global variable can be defined with var, let and const keywords if you are declaring them outside of any blocks which means at the top level of your program. And if you want to define a variable inside a block and you want to access it outside that block, you can do that with the var variable declaration. Because the var is a global scope in JavaScript.

This is what a global variable looks like, as shown in the following example.

const variableGlobal = "Amol Shelke";
console.log(variableGlobal);
Code language: JavaScript (javascript)

Examples of global variables and their use cases

Global variables are really helpful if you want to use a value again and again in your program as shown in the following example.

I have declared two global variables in the program below name and useAge, and depending on their values I have created a program to print some funny messages. The advantage of using a global variable is that I can change the values in one place and the program will still print the messages.

const name = "AMOL SHLEKE";
const userAge = 19;

if (userAge < 18) {
  console.log(`Hey There ${name} I hope you are doing well`);
} else if (userAge === 18) {
  console.log(`Hey there ${name} congratulations on being 18`);
} else {
  console.log(`${name} you are not teen anymore🥲`);
}
Code language: JavaScript (javascript)

Definition of Local Variables

Explanation of what a local variable is

A local variable in JavaScript is a variable that is limited to the block in which it has been declared, which means we cannot access that variable outside of the block in which it has been initialized. An example of a local variable is a variable which is declared inside the function, if-else block, while-loop/for-loop etc.

How local variables are defined and accessed in JavaScript

To define a local variable in JavaScript we can use the var, let and const keyword. A local variable in JavaScript can be defined in a block such as a function and if-else. Below is an example of the local variable created inside a function in JavaScript.


const localVarExample = () => {
  var nameLocal = "Amol Shelke"; // nameLocal is the local variable here
}
Code language: JavaScript (javascript)

Examples of local variables and their use cases

The local variables are used when we don’t want to use a variable outside any function in your program, in short, local variables can be used as private variables. As a developer, you have to create variables and use them so keep it in mind when you want a value that is only needed at a certain place use local variables.

Example with the use cases.

function makePizza() {
 const foodIngredients = "Cheeze, meonies, vegies and flour";
 return `Your pizza is ready with ${foodIngredients} `;
}
console.log(makePizza());
Code language: JavaScript (javascript)

In the above example, you can see the foodIngredients variable is only needed in the makePizza function that’s why I created a local variable inside the makePizza function. Because we only want the ingredients inside the function.

Differences between Global and Local Variables

Comparison of the scope, accessibility, and lifespan of global and local variables

Variables	Scope	Accessibility	Lifespan
Global variables.	The scope of global variables is globally scoped because they can be accessed wherever in the program. As compared to the local variables.	The global variables can be accessed wherever in the program in which it has been defined. One can use var, let and const to declare the global variables.	The lifespan of the global variables is until the program is completed executing which means until the web app is open in the browser the global variables will not gonna go.
Local variables.	The scope of local variables is locally scoped because they cannot be accessed wherever in the program. As compared to the global variables.	The local variables can’t be accessed wherever in the program, they can only be accessed inside the function/block in which it has been defined. One can use var, let and const to declare the global variables.	The lifespan of the local variables is until the function execution as soon as the function execution is done, The local variable will be gone.

Comparison between local and global variables.

Explanation of how these differences affect the way the variables are used in JavaScript

The scope of the local and global variables affects the way of declaring these variables in JavaScript. If you created a variable inside the function and wanted to use it outside you cannot do that. You will have to create a global variable in that case.
The Global variables can be accessed anywhere in the program so if we create another variable with the same name the accessibility issue will occur. And we cannot be able to create a variable with the same name.
The lifespan of a local variable will be until the function execution, as soon as the function gets executed the variable will be gone from the memory. So we cannot use that variable after the lifespan of the variable.

Best Practices for Using Global and Local Variables

Tips for choosing the appropriate type of variable for a given situation

Choose the global variables when you want to use that variable anywhere in your program.
Choose the local variables when you want to use the value of a variable only at a specific function/block.
If the value of a variable needs to be private and cannot be accessed from outside then choose local variables.

Suggestions for avoiding common mistakes and pitfalls when using global and local variables

Avoid using the let and var while creating global variables because as you know that in JavaScript let and var can be resigned, so if you created a variable with var or let and you changed the value of that variable inside any function it will affect the global variables. So make sure to use const as much as you can.
For local variables as you already know where the local variable is accessible so don’t create any unused local variables if there is no need for them inside the functions.

Conclusion

Summary of the main points covered in the blog

That’s it for this article, we covered everything about local and global variables of JavaScript, starting from definition to tips to use them in the program. I hope you found this article helpful. JavaScript local and global variables are really important topics which many new developers failed to learn, below are some of my favourite resources which I used to learn JavaScript from scratch. I hope this will help you too.

Additional resources for further reading and learning about global and local variables in JavaScript

To read more about the global and local variables of JavaScript you can always refer to the MDN where all the documentation is written in a human-written way. And if you are a tutorial kind of guy then watching Namaste JavaScript can be the best choice you can make.

FAQs

What are some best practices for using global and local variables in JavaScript?

Choose the global variables when you want to use that variable anywhere in your program.
Choose the local variables when you want to use the value of a variable only at a specific function/block.

How do the scope, accessibility, and lifespan of global and local variables affect their use in JavaScript?

The answer to this question is already written above in the blog.

Are there any specific use cases for global or local variables in JavaScript?

Yes, there are and it can be written in one line use the global variable when you want a variable present anywhere in your program And use the local variable when you don’t want your variable to be accessed outside of a function.

What are the common mistakes to avoid when using the global and local variables in JavaScript?