JavaScript Variables

JavaScript Variables are the building blocks of any programming language. In JavaScript, variables can be used to store reusable values. The values of the variables are allocated using the assignment operator(“=”).

JavaScript Identifiers

JavaScript variables must have unique names. These names are called Identifiers.

There are some basic rules to declare a variable in JavaScript:

• These are case-sensitive
• Can only begin with a letter, underscore(“_”) or “$” symbol
• It can contain letters, numbers, underscore, or “$” symbol
• A variable name cannot be a reserved keyword.

JavaScript is a dynamically typed language so the type of variables is decided at runtime. Therefore there is no need to explicitly define the type of a variable. We can declare variables in JavaScript in three ways:

• JavaScript var keyword
• JavaScript let keyword
• JavaScript const keyword 

Note: In JavaScript, variables can be declared automatically.

Syntax:

var geek = "Hello Geek"         // Declaration using var
let $ = "Welcome"                   // Declaration using let
const _example = "Gfg"       // Declaration using const   

All three keywords do the basic task of declaring a variable but with some differences Initially, all the variables in JavaScript were written using the var keyword but in ES6 the keywords let and const were introduced.

Example 1: In this example, we will declare variables using var.

Output:

Hello Geeks
10
12
22

Example 2: In this example, we will declare variables using let.

Hello learners
joining
 12
joining 12

To learn more about JavaScript let check this article JavaScript Let

Example 3: In this example, we will declare the variable using the const keyword.

Output:

Explanation: const keyword is used when we assign a value permanently to a variable. So when we try to change the value of a variable declared with the const keyword it will throw an error. The variables declared with var and let are mutable that is their value can be changed but variables declared using const are immutable. 

To learn more about JavaScript const check this article JavaScript Const

Note: The newly introduced keywords let and const are block scoped whereas var is function scoped. 

Let us see an example to understand the difference:

Example: In this example, we are trying to access the block scoped variables outside the block that’s why we are getting error.

Output:

Explanation: Since variables “a” and “c” are block scoped so we were not able to access them outside their block.

When to Use var, let, or const

• We declare variables using const if the value should not be changed
• We use const if the type of the variables should not be changed such as working with Arrays and objects
• We should use let if not using const

To learn more about the scope of variables refer to this article Understanding variable scopes in JavaScript

Comparison of properties of let, var, and const keywords in JavaScript: