JavaScript Variables

Storing and managing data in JavaScript

📦 What are JavaScript Variables?

Variables are containers that store data values. In JavaScript, you can create variables using var, let, or const keywords. 


// Creating variables
let name = "John";
const age = 25;
var city = "New York";

console.log(name, age, city);

Output:

John 25 New York

Variable Declaration Keywords

let

let

Block-scoped, can be reassigned 

let x = 10;
x = 20; // Can change
const

const

Block-scoped, cannot be reassigned 

const PI = 3.14159;
// PI = 3.14; // Error!
var

var

Function-scoped, can be reassigned 

var name = "John";
name = "Jane"; // Can change
🏷️

Naming

Rules for variable names 

let myVariable = 1;
let _private = 2;
let $special = 3;

🔹 Variable Declaration and Assignment

You can declare variables and assign values in different ways: 

// Declaration and assignment together
let firstName = "John";
const lastName = "Doe";

// Declaration first, assignment later
let age;
age = 25;

// Multiple variables at once
let x = 5, y = 10, z = 15;

console.log(firstName, lastName, age, x, y, z);

Output:

John Doe 25 5 10 15

🔹 Variable Naming Rules

JavaScript variable names must follow certain rules: 

// Valid variable names
let userName = "John";
let user_name = "Jane";
let $price = 100;
let _count = 5;
let age2 = 25;

// Invalid variable names (will cause errors)
// let 2age = 25;        // Cannot start with number
// let user-name = "X";  // Cannot contain hyphens
// let let = "value";    // Cannot use reserved words

console.log(userName, user_name, $price, _count, age2);

Output:

John Jane 100 5 25

🔹 Variable Scope

Variables have different scopes depending on where they are declared: 

// Global scope
let globalVar = "I'm global";

function myFunction() {
  // Function scope
  let functionVar = "I'm in function";
  
  if (true) {
    // Block scope
    let blockVar = "I'm in block";
    console.log(globalVar);    // Accessible
    console.log(functionVar);  // Accessible
    console.log(blockVar);     // Accessible
  }
  
  // console.log(blockVar);  // Error! Not accessible here
}

myFunction();

Output:

I'm global
I'm in function
I'm in block

🧠 Test Your Knowledge

Which keyword creates a variable that cannot be reassigned?