Variables and constants

In JavaScript, variables are used to store data values, and constants hold values that do not change. There are three ways to declare variables: var, let, and const.

1. Declaring Variables

JavaScript provides two modern ways (let and var) and one older way (var) to declare variables.

1.1 Using var (Old Way, Avoid Using)

• Can be redeclared within the same scope.
• Function-scoped (not block-scoped).
• May lead to unexpected behavior due to hoisting.

var name = "Alice"; 
console.log(name); // Output: Alice
var name = "Bob"; // Redeclaration is allowed
console.log(name); // Output: Bob

🔹 Why avoid var?

Since var does not respect block scope, it can cause unexpected issues in larger programs.

1.2 Using let (Modern & Recommended)

Block-scoped (only available inside {} where declared).

Can be updated but not redeclared in the same scope.

let age = 25;
console.log(age); // Output: 25
age = 30; // Allowed: Updating the variable
console.log(age); // Output: 30
// let age = 40; ❌ Error: Cannot redeclare the variable

🔹 Best practice: Use let for values that may change.

2. Declaring Constants (const)

• Block-scoped like let.
• Cannot be reassigned after initialization.
• Must be initialized when declared.

const PI = 3.14;
console.log(PI); // Output: 3.14
// PI = 3.14159; ❌ Error: Cannot reassign a constant

🔹 Best practice: Use const for values that should not change (e.g., mathematical constants, API keys).

Scope of Variables

JavaScript variables have different scopes depending on how they are declared: