Q274- What is the difference between var, let, and const? Explain scope, hoisting, and re-declaration with examples.

JavaScript provides three ways to declare variables.
They differ in scope, hoisting behavior, and re-declaration rules.

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✅ 1. Scope

🔹 var → Function-scoped

if (true) {
  var x = 10;
}
console.log(x); // 10 ❌ (leaks outside block)

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🔹 let → Block-scoped

if (true) {
  let y = 20;
}
console.log(y); // ReferenceError ✔

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🔹 const → Block-scoped

if (true) {
  const z = 30;
}
console.log(z); // ReferenceError ✔

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✅ 2. Hoisting

All three are hoisted, but behave differently.

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🔹 var → hoisted with undefined

console.log(a); // undefined ❌
var a = 5;

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🔹 let & const → hoisted but in Temporal Dead Zone (TDZ)

console.log(b); // ReferenceError ✔
let b = 10;

console.log(c); // ReferenceError ✔
const c = 15;

✔ Cannot be accessed before declaration

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✅ 3. Re-declaration

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🔹 var → allowed (dangerous)

var x = 1;
var x = 2; // allowed ❌

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🔹 let → NOT allowed

let y = 1;
let y = 2; // Error ✔

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🔹 const → NOT allowed

const z = 1;
const z = 2; // Error ✔

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✅ 4. Re-assignment

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🔹 var → allowed

var a = 10;
a = 20;

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🔹 let → allowed

let b = 10;
b = 20;

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🔹 const → NOT allowed

const c = 10;
c = 20; // Error ❌

⚠️ But objects & arrays can be mutated:

const user = { name: "Teekam" };
user.name = "TS"; // allowed ✔

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🔥 Summary Table

Feature	var	let	const
Scope	Function	Block	Block
Hoisting	Yes (undefined)	Yes (TDZ)	Yes (TDZ)
Re-declaration	✔ Allowed	❌ Not allowed	❌ Not allowed
Re-assignment	✔ Allowed	✔ Allowed	❌ Not allowed
Default use	❌ Avoid	✔ Use	✔ Use

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🎯 Short Interview Answer

var is function-scoped, hoisted with undefined, and allows re-declaration, which can cause bugs.
let and const are block-scoped, hoisted but placed in the Temporal Dead Zone, preventing access before declaration.
let allows reassignment, while const does not, making them safer and preferred in modern JavaScript.

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⭐ Best Practice

✔ Use const by default
✔ Use let only when reassignment is required
❌ Avoid var in modern code