Let’s go through const in JavaScript in detail.

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🔹 const in JavaScript

The const keyword is used to declare constants in JavaScript.
👉 Introduced in ES6 (2015), just like let.

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1️⃣ Syntax

const variableName = value;

Example:

const PI = 3.14159;
const name = "Raj";

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2️⃣ Features of const

✅ Block Scope

• Just like let, const is block-scoped.

{
  const x = 10;
  console.log(x); // 10
}
// console.log(x); ❌ Error (not accessible outside block)

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❌ Cannot Redeclare or Reassign

• You cannot redeclare or reassign a const variable.

const city = "Delhi";
// city = "Mumbai"; ❌ Error: Assignment to constant variable
// const city = "Pune"; ❌ Error: Already declared
console.log(city); // Delhi

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✅ Must Be Initialized

• You must assign a value when declaring a const.

// const x; ❌ Error: Missing initializer
const x = 50; // ✅ Correct

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✅ Hoisting with TDZ

• Like let, const is hoisted but exists in the Temporal Dead Zone (TDZ) until declared.
• Accessing it before declaration throws an error.

// console.log(a); ❌ ReferenceError
const a = 10;

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✅ Objects & Arrays with const

• Important: const makes the binding (variable reference) constant, not the actual value inside objects/arrays.
• You can modify contents of objects/arrays declared with const.

// Array Example
const numbers = [1, 2, 3];
numbers.push(4); // ✅ Allowed
console.log(numbers); // [1, 2, 3, 4]
// numbers = [5, 6]; ❌ Error (cannot reassign whole array)

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

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3️⃣ When to Use const?

• When you know a value should never change.
• Good for:
  • Constants (PI, tax rates, app settings).
  • Arrays/objects whose reference shouldn’t change.

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✅ Summary

• Block-scoped (like let).
• Must be initialized when declared.
• ❌ Cannot redeclare or reassign.
• ✅ Can modify objects/arrays (but not reassign them).
• Preferred by default in modern JS (use let only if reassignment is needed).