JavaScript Variables and Constants

JavaScript Variables

A JavaScript variable is a container for storing data. For example,

let num = 5;

Here, num is a variable that stores the number 5.


Declare Variables in JavaScript

In JavaScript, we use the var or let keywords to declare variables. For example,

var age;
let name;

Here, age and name are variables.

What is the difference between var and let?

Both var and let are used to declare variables. However, there are some differences between them.

var let
var is used in older versions of JavaScript. let is the new way of declaring variables, starting with ES6 (ES2015).
Variables created with var are function-scoped, meaning they can be accessed anywhere within the function they were defined in. Variables declared with let are block-scoped, meaning they can only be accessed within the block where they were declared.
For example, var x; For example, let y;

To learn more, visit JavaScript let Vs var.

Note: It is recommended we use let instead of var. However, there are a few browsers that do not support let. Visit JavaScript let browser support to learn more.


Initialize Variables in JavaScript

We use the assignment operator = to assign a value to a variable.

// declare variable num
let num;

// assign 5 to num
num = 5;

Here, 5 is assigned to the variable num.

You can also initialize variables during its declaration.

// declare variable num1 and assign 5 to it
let num1 = 5;

// declare variable num2 and assign 6 to it
let num2 = 6;
Declare multiple variables in a single statement.

In JavaScript, it's possible to declare multiple variables in a single statement.

// declare variables num1, num2, and num3
// assign values 5, 6, and 7 respectively
let num1 = 5, num2 = 6, num3 = 7;

Here, we have declared and assigned values to three variables in a single line:

  • The value assigned to num1 is 5.
  • The value assigned to num2 is 6.
  • The value assigned to num3 is 7.
Use a variable without initializing it.

If you use a variable without initializing it, it will have an undefined value.

// declare variable num
let num; 

// print num
console.log(num); 

// Output: undefined

Here, we have declared a variable named num. However, since it does not contain any value, its value is undefined.

To learn more about undefined, visit JavaScript null and undefined.


Change the Value of Variables

The value of a variable may vary. Hence, the name variable.

Let's look at the example below to learn how to change the value of a variable:

// assign 5 to variable score
let score = 5; 
console.log(score); // 5

// change the value of score to 3
score = 3; 
console.log(score); // 3

Here, the value of the score variable is changed from 5 to 3 when we assign a new value to it.


Rules for Naming JavaScript Variables

  • Variable names must start with a letter, an underscore _, or the dollar sign $. For example,
// valid
let message = "hello";
let _message = "hello";
let $message = "hello";
  • Variables cannot start with numbers. For example,
// invalid
let 1message = "hello"; // this gives an error
  • Variable names are case-sensitive. So age and Age are different variables. For example,
let age = 23;
let Age = 20;

console.log(age); // 23
console.log(Age); // 20
  • Variable names cannot be keywords (special words reserved for specific purposes in JavaScript such as if, else, let, var, etc.). For example,
//invalid
let new = 5; // Error! new is a keyword
Recommended ways to name a variable in JavaScript.

You can name the variables any way you want. However, we recommend you use the following naming conventions:

  • In JavaScript, variables are generally named in camelCase format if they have multiple words.

    For example, firstName, annualSalary, numberOfBooks, etc.
  • It's a good practice to give a descriptive name to a variable.

    For example, if you are using a variable to store the number of apples, it is better to name that variable apples or numberOfApples rather than x or n.

JavaScript Constants

A constant is a type of variable whose value cannot be changed.

In JavaScript, we use the const keyword to create constants. For example,

// assign 5 to num 
const num = 5;

Once a constant is initialized, we cannot change its value.

// assign 5 to num
const num = 5;

// assign 10 to num
num = 10;  
console.log(num) // Error! constant cannot be changed

Always Initialize a Constant During Declaration

If you do not initialize a constant at the time of declaration, it throws an error. For example,

// Error! Missing initializer in const declaration
const x;

// attempt to initialize constant after declaration
x = 5;

console.log(x)

Note: If you are sure that the value of a variable won't change throughout the program, we recommend you use const.

However, there are a few browsers that do not support const. Visit JavaScript const browser support to learn more.


Also Read

Video: JavaScript Variables

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