JavaScript Variable Declarations -let, const & var

🧠 let, const, and var**

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🔸 var – The Old Way (ES5)

• Introduced in early versions of JavaScript.
• Function-scoped
• Can be redeclared and updated
• Hoisted (moved to the top of the scope)

✅ Example:

var x = 10;
var x = 20; // No error (redeclared)
console.log(x); // 20

⚠️ Problem:

if (true) {
  var test = "hello";
}
console.log(test); // hello (leaks outside the block)

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🔹 let – Modern Way (ES6)

• Introduced in ES6 (2015)
• Block-scoped
• Can be updated but not redeclared in the same scope
• Not hoisted (in a usable way)

✅ Example:

let a = 5;
a = 10; // OK
// let a = 15; ❌ Error: Identifier 'a' has already been declared

if (true) {
  let msg = "inside";
  console.log(msg); // inside
}
// console.log(msg); ❌ Error: msg is not defined

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🔒 const – For Constants (ES6)

• Also block-scoped
• Cannot be updated or redeclared
• Must be initialized at the time of declaration

✅ Example:

const pi = 3.14;
// pi = 3.14159; ❌ Error: Assignment to constant variable

⚠️ Objects with const:

You can change object properties, but not reassign the object.

const person = { name: "Sudhakar" };
person.name = "SK";  // ✅ OK
// person = {}; ❌ Error

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📊 Comparison Table

Feature	var	let	const
Scope	Function	Block	Block
Hoisting	Yes (initialized as undefined)	Yes (but not initialized)	Yes (but not initialized)
Re-declaration	✅ Allowed	❌ Not allowed	❌ Not allowed
Re-assignment	✅ Allowed	✅ Allowed	❌ Not allowed
Initialization	Optional	Optional	✅ Required
Use Case	Legacy code	Variables that change	Fixed values / constants

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✅ Best Practices

• Use const by default
• Use let when the value needs to change
• Avoid var (use only in legacy projects)