1️⃣ let & const

🔹 let

• Block-scoped (only works inside { })
• Can be reassigned

let age = 25;
age = 30; // ✅ allowed

🔹 const

• Block-scoped
• Cannot be reassigned (but objects/arrays can be modified)

const name = "Aditya";
// name = "Rahul"; ❌ errorconst user = { city: "Delhi" };
user.city = "Mumbai"; // ✅ allowed

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2️⃣ Arrow Functions (=>)

Shorter way to write functions.

Normal Function

function add(a, b) {
  return a + b;
}

Arrow Function

const add = (a, b) => a + b;

With one parameter

const greet = name => "Hello " + name;

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3️⃣ Template Literals ( )

Use backticks instead of quotes → allows variables & multi-line strings.

let name = "Aditya";
let msg = `Hello ${name}, welcome!`;console.log(msg);

Multi-line

let text = `Line 1
Line 2
Line 3`;

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4️⃣ Destructuring

Extract values from arrays/objects easily.

Array Destructuring

let arr = [10, 20, 30];
let [a, b] = arr;console.log(a); // 10

Object Destructuring

let user = { name: "Aditya", age: 22 };let { name, age } = user;

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5️⃣ Spread Operator (...)

Expands elements (used to copy or merge).

Array Example

let arr1 = [1, 2];
let arr2 = [...arr1, 3, 4];console.log(arr2); // [1,2,3,4]

Object Example

let user = { name: "Aditya" };
let updated = { ...user, city: "Delhi" };

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6️⃣ Rest Operator (...)

Collects multiple values into one variable.

function sum(...numbers) {
  return numbers.reduce((total, num) => total + num);
}console.log(sum(1,2,3,4)); // 10

👉 Difference:

• Spread → expands
• Rest → collects

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7️⃣ Modules (import / export)

Used to split code into multiple files.

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Export (file: math.js)

export const add = (a, b) => a + b;
export const sub = (a, b) => a - b;

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Import (file: app.js)

import { add, sub } from './math.js';console.log(add(5, 3));

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Default Export

// math.js
export default function multiply(a, b) {
  return a * b;
}

// app.js
import multiply from './math.js';

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🔥 Quick Summary

Feature	Use
let	variable (changeable)
const	fixed variable
Arrow function	shorter functions
Template literals	dynamic strings
Destructuring	extract values
Spread	expand data
Rest	collect data
Modules	split code