Your guide to the JavaScript programming language

Variables & Constants in JavaScript

JavaScript is a versatile programming language that allows you to create dynamic and interactive web applications. One of the core concepts of any programming language is variables, which enable you to store and manipulate data in your code.

In this post, we'll explore what variables are, how to declare and assign values to them, and some best practices for working with variables in JavaScript.

What Are Variables?

Variables are containers that hold values of different types, such as numbers, strings (or text), or boolean values (logical values of true and false). You can think of them as named locations for storing data. For example, x = 10 where x is a variable and it stores the value 10 currently. These values can be used and manipulated throughout your code, making it more flexible and dynamic.

In JavaScript, you can declare variables using the var, let, or const keywords. Each of these keywords has a different scope and behavior, which we'll explore in more detail later in this post.

Declaring and Assigning Values to Variables

To declare a variable, you simply need to use one of the keywords (var, let, or const) followed by the variable name. For example, to declare a variable named age, you can use the following code:

var age;

Once you've declared a variable, you can assign a value to it using the = operator. For example, to assign the value 30 to the age variable, you can use the following code:

age = 30;

You can also assign a value to a variable at the time of declaration, like this:

var name = "John";

You can even use a variable to assign value to another variable.

var greeting = "Hello World!";
var hello = greeting;

Similarly, you can also use the let keyword to create variable. For example:

let age = "30";
let name;
name = "John";

JavaScript allows you to create and define multiple variables on the same line.

let x, y;
let a, b = 4, 6;
x, y = true, true;

There's a slight difference between the scopes of the variables created by var and let. However, we'll cover that in a future post. For now, you can assume that you can use them interchangeably.


Sometimes you may have a value that you don't want to change throughout your code. That's where constants come in.

Constants in JavaScript are variables whose value cannot be changed once they are defined. This means that they are read-only and cannot be reassigned or modified.

To declare a constant in JavaScript, you use the const keyword followed by the variable name and the initial value:

const pi = 3.14;

In this example, pi is a constant that is initialized with the value 3.14. You must always assign a value at the time of declaring a constant. Once pi is defined, you cannot change its value.

pi = 3; // Uncaught TypeError: Assignment to constant variable.

const x; // Uncaught SyntaxError: Missing initializer in const declaration

Constants are useful for storing values that should not be changed throughout your code, such as mathematical constants like pi, or configuration values like URLs and API keys.

Best Practices

Here are a few best practices for working with variables in JavaScript:

  • Use descriptive variable names that accurately reflect their purpose. This makes it easier for other developers (and yourself!) to understand your code.
  • Use const for values that should not change throughout your code, and use let/var for values that can change.
  • In JavaScript, it's common to use camelCase for variable names. This means that the first word is lowercase and subsequent words are capitalized. For example, firstName or userAge.

I hope you found this post helpful!