Let’s make it super clear by comparing var, let, and const in JavaScript.

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📊 var vs let vs const

Feature	var	let	const
Scope	Function-scoped	Block-scoped { }	Block-scoped { }
Hoisting	✅ Hoisted (initialized as undefined)	✅ Hoisted (but TDZ → error if accessed before declaration)	✅ Hoisted (but TDZ → error if accessed before declaration)
Redeclaration	✅ Allowed	❌ Not allowed	❌ Not allowed
Reassignment	✅ Allowed	✅ Allowed	❌ Not allowed
Initialization	Optional (defaults to undefined)	Optional	Mandatory (must assign a value)
Use in Loops	Problematic (no block scope)	Safe	Safe
Best Use Case	Old/legacy code	Variables that may change	Constants / Fixed values

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🔹 Examples

1️⃣ var Example (Function-scoped, Redeclarable)

var x = 10;
var x = 20;   // ✅ Redeclaration allowed
console.log(x); // 20

⚠ Problem: It leaks outside block scope

if (true) {
  var city = "Delhi";
}
console.log(city); // ✅ Works (not block-scoped, unsafe)

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2️⃣ let Example (Block-scoped, Reassignable)

let age = 20;
age = 21;      // ✅ Reassignment allowed
// let age = 22; ❌ Error (no redeclaration in same scope)
console.log(age); // 21

Safe in loops:

for (let i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 1000);
}
// ✅ Output: 0, 1, 2

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3️⃣ const Example (Block-scoped, Fixed Reference)

const PI = 3.14159;
// PI = 3.15; ❌ Error (cannot reassign)
console.log(PI); // 3.14159

But objects/arrays inside const can still be modified:

const person = { name: "Raj", age: 20 };
person.age = 21; // ✅ Allowed
// person = {};   ❌ Error (cannot reassign object reference)
console.log(person); // { name: 'Raj', age: 21 }

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🎯 Real-Life Analogy

• var → Old container with holes (not safe, things leak out).
• let → New box you can reopen and change items inside.
• const → Sealed box, you cannot replace it, but you can rearrange items inside (for objects/arrays).

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✅ Which One Should You Use?

• Use const by default.
• Use let if you know the value will change.
• Avoid var (only for legacy/old code).