JavaScript Variables

In this chapter, we will learn about JavaScript variables. We will cover:

• What is a Variable?
• Declaring Variables
• Variable Types
• Variable Scope
• Variable Naming Rules

What is a Variable?

A variable is a container for storing data values. Think of it as a box where you can put information that you want to use later. In JavaScript, you can store numbers, text, objects, and more in variables.

Declaring Variables

In JavaScript, you can declare variables using var, let, or const.

Using var

var is used to declare variables that can be re-declared and updated. It has function scope, meaning it is accessible throughout the function in which it is declared.

Example:

var name = "Amit"; console.log(name); // Output: Amit var name = "Raj"; console.log(name); // Output: Raj name = "Priya"; console.log(name); // Output: Priya 

Using let

let is used to declare variables that can be updated but not re-declared within the same scope. It has block scope, meaning it is only accessible within the block (e.g., {}) where it is declared.

Example:

let age = 25; console.log(age); // Output: 25 age = 26; console.log(age); // Output: 26 // let age = 30; // This will cause an error if uncommented 

Using const

const is used to declare variables that cannot be updated or re-declared. It also has block scope. Once a value is assigned to a const variable, it cannot be changed.

Example:

const pi = 3.14; console.log(pi); // Output: 3.14 // pi = 3.14159; // This will cause an error if uncommented // const pi = 3.15; // This will also cause an error if uncommented 

Variable Types

JavaScript variables can hold different types of data:

Numbers

Example:

let number = 10; console.log(number); // Output: 10 number = 15.5; console.log(number); // Output: 15.5 

Strings

Example:

let greeting = "Namaste!"; console.log(greeting); // Output: Namaste! greeting = 'Hello, JavaScript!'; console.log(greeting); // Output: Hello, JavaScript! 

Booleans

Example:

let isStudent = true; console.log(isStudent); // Output: true isStudent = false; console.log(isStudent); // Output: false 

Arrays

Example:

let colors = ["red", "green", "blue"]; console.log(colors); // Output: ["red", "green", "blue"] colors[0] = "yellow"; console.log(colors); // Output: ["yellow", "green", "blue"] 

Objects

Example:

let person = { firstName: "Ravi", lastName: "Kumar", age: 25 }; console.log(person); // Output: { firstName: "Ravi", lastName: "Kumar", age: 25 } person.age = 26; console.log(person); // Output: { firstName: "Ravi", lastName: "Kumar", age: 26 } 

Null and Undefined

Example:

let emptyValue = null; console.log(emptyValue); // Output: null let undefinedValue; console.log(undefinedValue); // Output: undefined 

Variable Scope

Scope determines the visibility of variables. JavaScript has two types of scope:

Function Scope

Variables declared with var inside a function are function-scoped.

Example:

function myFunction() { var message = "Hello, Function Scope!"; console.log(message); // Output: Hello, Function Scope! } // console.log(message); // This will cause an error if uncommented 

Block Scope

Variables declared with let or const inside a block (e.g., inside curly braces {}) are block-scoped.

Example:

if (true) { let blockMessage = "Hello, Block Scope!"; console.log(blockMessage); // Output: Hello, Block Scope! } // console.log(blockMessage); // This will cause an error if uncommented 

Variable Naming Rules

• Variable names must start with a letter, underscore (_), or dollar sign ($).
• Variable names can contain letters, digits, underscores, and dollar signs.
• Variable names are case-sensitive (myVariable and myvariable are different).

Example:

let _name = "Ravi"; let $age = 25; let fullName = "Ravi Kumar"; console.log(_name); // Output: Ravi console.log($age); // Output: 25 console.log(fullName); // Output: Ravi Kumar 

Conclusion

In this chapter, you learned about declaring variables using var, let, and const, different types of variables, variable scope, and naming rules. Understanding these basics is crucial for writing clear and efficient JavaScript code. In the next chapter, we will explore JavaScript data types and how to use them in your JavaScript programs.