#165 var vs let vs const in JavaScript

🔹 var vs let vs const in JavaScript

Feature	var	let	const
Scope	Function-scoped	Block-scoped ({})	Block-scoped ({})
Hoisting	✅ Hoisted → initialized as undefined	✅ Hoisted → but in Temporal Dead Zone (TDZ) until initialized	✅ Hoisted → but in TDZ until initialized
Re-declaration	✅ Allowed in same scope	❌ Not allowed in same scope	❌ Not allowed
Re-assignment	✅ Allowed	✅ Allowed	❌ Not allowed (must assign value at declaration)
Global object property	✅ Attached to window (in browsers)	❌ Not attached	❌ Not attached
Best use case	Legacy code (avoid using in modern JS)	Variables that will change values	Constants that never change (IDs, config, etc.)

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🔹 Examples

1. Scope Difference

if (true) {
  var x = 10;
  let y = 20;
  const z = 30;
}

console.log(x); // ✅ 10 (var is function-scoped)
console.log(y); // ❌ ReferenceError (block-scoped)
console.log(z); // ❌ ReferenceError (block-scoped)

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2. Hoisting

console.log(a); // ✅ undefined (hoisted with default undefined)
var a = 5;

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

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

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3. Re-declaration & Re-assignment

// var
var name = "Dev";
var name = "John"; // ✅ allowed
console.log(name); // John

// let
let age = 25;
// let age = 30; // ❌ SyntaxError
age = 30;        // ✅ allowed

// const
const city = "Delhi";
// const city = "Mumbai"; // ❌ SyntaxError
// city = "Mumbai";       // ❌ TypeError

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4. Global Object Property (browser example)

var a = 100;
let b = 200;
const c = 300;

console.log(window.a); // ✅ 100
console.log(window.b); // ❌ undefined
console.log(window.c); // ❌ undefined

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🔹 Summary

• var → function-scoped, hoisted, re-declarable → ❌ avoid in modern JS.
• let → block-scoped, re-assignable, safer → ✅ use for variables that change.
• const → block-scoped, immutable binding, must initialize → ✅ use for constants.