• A variable is a named storage location in computer memory
• Used to store and manage data during program execution
• Acts as a container that holds a specific value which can be modified or referenced

• Provides a way to label and store data
• Enables dynamic data manipulation
• Supports various data types and scopes

var oldStyleVariable = "I'm an old school variable";

• Function-scoped or globally-scoped
• Can be re-declared and updated
• Hoisted to the top of its scope
• Less predictable behavior
• Not Recommended in Modern JavaScript

console.log(x); // Outputs: undefined (not an error)
var x = 5;

let modernVariable = "I'm a block-scoped variable";

• Block-scoped (limited to {} block)
• Can be updated but not re-declared in same scope
• More predictable behavior
• Prevents unintended global variable creation
• Recommended for Mutable Variables

{
    let blockVariable = "I exist only in this block";
}
// blockVariable is not accessible outside the block

const unchangeableValue = "I cannot be reassigned";

• Block-scoped
• Cannot be reassigned after initial value
• Prevents accidental variable reassignment
• Recommended as Default Declaration

const person = { name: "Pankaj" };
person.name = "Updated Name"; // Allowed
// person = {}; // This would cause an error

• Must start with a letter, underscore (_), or dollar sign ($)
• Can contain letters, numbers, underscores, dollar signs
• Case-sensitive (age and Age are different variables)
• No spaces or special characters allowed

• Use camelCase for variable names
• Choose descriptive and meaningful names
• Avoid single-letter names (except in specific contexts like loops)

let firstName = "Pankaj";
let totalStudentCount = 100;
let isLoggedIn = true;

let 2name = "Invalid";        // Starts with number
let user-name = "Not Good";   // Contains hyphen
let class = "Reserved word";  // Reserved keyword

1. String: Text data

   let greeting = "Hello, World!";

2. Number: Numeric values

   let age = 30;
   let temperature = 98.6;

3. Boolean: True/False values

   let isStudent = true;

4. Undefined: Declared but not assigned

   let undefinedVar;

5. Null: Intentional absence of any object value

   let emptyValue = null;

6. Symbol: Unique identifier (ES6)

   let uniqueKey = Symbol('description');

7. BigInt: For very large integers

   let bigNumber = 1234567890123456789012345678901234567890n;

1. Object: Key-value collections

   let person = {
       name: "Pankaj",
       age: 30,
       skills: ["JavaScript", "React"]
   };

2. Array: Ordered list of values

   let fruits = ["Apple", "Banana", "Cherry"];

• Variables declared outside any function
• Accessible everywhere in the code

• Variables declared inside a function
• Accessible only within that function

• Variables declared inside {} blocks
• Limited accessibility

let globalVar = "Global";

function exampleScope() {
    let functionVar = "Function Scope";
    
    if (true) {
        let blockVar = "Block Scope";
        console.log(globalVar);     // Accessible
        console.log(functionVar);   // Accessible
        console.log(blockVar);      // Accessible
    }
    
    // console.log(blockVar);  // Would cause an error
}

1. Use const by default
2. Use let when value will change
3. Avoid var in modern JavaScript
4. Initialize variables when declaring
5. Group related declarations together

• Declare variables as close to usage as possible
• Release references when no longer needed
• Be mindful of variable lifetime

let result = "5" + 3;  // Becomes "53" (string concatenation)
let math = "5" - 3;    // Becomes 2 (numeric subtraction)

let stringNum = "100";
let number = Number(stringNum);    // Converts to number
let string = String(number);       // Converts to string
let boolean = Boolean(stringNum);  // Converts to boolean

Understanding variables is fundamental to JavaScript programming. Practice and consistent use will make these concepts second nature.

• const: Unchanging values
• let: Changing values
• var: Avoid in modern code

Happy Coding! 🚀👨‍💻