In modern JavaScript and especially while working with React, understanding how functions behave internally helps us write clean and bug-free code. In this article, we'll cover:

• Closures

• Rest Parameters

• Default Parameters

• Hoisting in Functions

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✅ 1. Closures

📘 What is a Closure?

A closure is a function that remembers variables from its outer (parent) function, even after the parent function has finished executing.

It’s like a backpack that the inner function carries, which contains all the variables from the parent function.

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🔐 Real-life Example 1 – Counter Function

function createCounter() {
  let count = 0;
  return function () {
    count++;
    return count;
  };
}

const counter = createCounter();

console.log(counter()); // 1
console.log(counter()); // 2

✅ Here, the inner function remembers the count variable even after createCounter() has run.

---

🛒 Real-life Example 2 – Cart Items

function createCart() {
  let items = [];
  return function (item) {
    items.push(item);
    return items;
  };
}

const cart = createCart();

console.log(cart("Milk")); // ["Milk"]
console.log(cart("Bread")); // ["Milk", "Bread"]

✅ Even though createCart() is done, the inner function can still access items.

---

🔐 Real-life Example 3 – Personalized Greeter

function greetUser(name) {
  return function (message) {
    return `Hello ${name}, ${message}`;
  };
}

const greetPayal = greetUser("Payal");

console.log(greetPayal("welcome to our site!"));
// Output: Hello Payal, welcome to our site!

✅ The returned function keeps access to the name even outside the parent.

---

✅ 2. Rest Parameters

📘 What is a Rest Parameter?

The rest parameter allows a function to accept any number of arguments as an array using ... syntax.

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📊 Real-life Example 1 – Calculate Total Price

function totalPrice(...prices) {
  return prices.reduce((sum, price) => sum + price, 0);
}

console.log(totalPrice(100, 200, 50)); // Output: 350

---

🧮 Real-life Example 2 – Count Arguments

function countArgs(...args) {
  return `You passed ${args.length} arguments`;
}

console.log(countArgs("HTML", "CSS", "JS", "React")); 
// Output: You passed 4 arguments

---

🔧 Real-life Example 3 – Join Words

function joinWords(...words) {
  return words.join(" ");
}

console.log(joinWords("Frontend", "Development", "Rocks!")); 
// Output: Frontend Development Rocks!

✅ Use this when you don’t know in advance how many arguments will be passed.

---

✅ 3. Default Parameters

📘 What are Default Parameters?

Default parameters are used to give default values to function parameters if no value or undefined is passed.

---

💬 Real-life Example 1 – Greeting Message

function greet(name = "Guest") {
  console.log(`Hello, ${name}!`);
}

greet("Payal"); // Hello, Payal!
greet();        // Hello, Guest!

---

📦 Real-life Example 2 – Product with default quantity

function addToCart(product, quantity = 1) {
  console.log(`${product} added with quantity ${quantity}`);
}

addToCart("Milk");          // Milk added with quantity 1
addToCart("Rice", 3);       // Rice added with quantity 3

---

🎯 Real-life Example 3 – Calculator with default operator

function calculate(a, b, operator = "add") {
  if (operator === "add") return a + b;
  if (operator === "subtract") return a - b;
}

console.log(calculate(5, 3)); // 8
console.log(calculate(5, 3, "subtract")); // 2

✅ Saves time by avoiding extra checks or writing conditional logic for undefined values.

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✅ 4. Function Hoisting

📘 What is Function Hoisting?

In JavaScript, function declarations are hoisted — meaning they are moved to the top of their scope before the code runs.

So, you can call the function before it's defined.

---

⏫ Example 1 – Works fine (function declaration)

sayHello(); // Output: Hello!

function sayHello() {
  console.log("Hello!");
}

✅ Works because sayHello is hoisted.

---

❌ Example 2 – Not hoisted (function expression)

sayHi(); // ❌ Error: Cannot access before initialization

const sayHi = function() {
  console.log("Hi!");
};

❌ sayHi is not hoisted because it's a function expression, not a declaration.

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📦 Example 3 – Hoisting with variables and functions

console.log(product); // undefined (due to var hoisting)
var product = "Milk";

show(); // Output: Showing product
function show() {
  console.log("Showing product");
}

✅ Variables declared with var are also hoisted, but only their declaration, not the value.

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🔁 Summary Table

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✅ Final Tips

• Use closures when you want to maintain state or create private variables.

• Use rest parameters to handle dynamic inputs (e.g., user skills, cart items).

• Use default parameters to reduce code repetition and avoid undefined values.

• Understand hoisting well to avoid runtime errors — especially in React event handlers and lifecycle methods.

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✅ Mini Project: "Quiz App with Score Tracker"

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💡 Purpose:

This small app will:

• Display a question with options

• Track score using a closure

• Accept any number of questions using rest parameters

• Use default parameters to avoid missing data

• Demonstrate function hoisting with utility functions

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📁 Project Files:

• index.html — structure

• style.css — minimal styling

• script.js — full logic using closures, rest, default, hoisting

---

📄 1. index.html

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8" />
  <title>Quiz App</title>
  <link rel="stylesheet" href="style.css" />
</head>
<body>
  <div class="container">
    <h2>🧠 Quiz App</h2>
    <div id="questionBox">Loading question...</div>
    <div id="options"></div>
    <p>Score: <span id="score">0</span></p>
    <button id="nextBtn">Next</button>
  </div>

  <script src="script.js"></script>
</body>
</html>

---

🎨 2. style.css

body {
  font-family: sans-serif;
  background-color: #f2f2f2;
  display: flex;
  justify-content: center;
  padding-top: 50px;
}

.container {
  background: white;
  padding: 20px;
  border-radius: 10px;
  width: 350px;
  box-shadow: 0 0 10px rgba(0,0,0,0.1);
}

button {
  margin-top: 10px;
  padding: 10px;
  background: teal;
  color: white;
  border: none;
  cursor: pointer;
  width: 100%;
}

---

🧠 3. script.js

// ✅ Function hoisting (you can call setupQuiz before it's defined)
setupQuiz();

// ✅ Closure for score tracking
const scoreManager = (function () {
  let score = 0;
  return {
    increment: function () {
      score++;
    },
    getScore: function () {
      return score;
    }
  };
})();

// ✅ Quiz data using rest and default parameters
function getQuestions(...questions) {
  return questions.length ? questions : [
    {
      question: "What is JavaScript?",
      options: ["Programming Language", "Animal", "Toy"],
      answer: "Programming Language"
    }
  ];
}

// 🧠 Main quiz logic
function setupQuiz() {
  const questions = getQuestions(
    {
      question: "What does HTML stand for?",
      options: ["Hyper Text Markup Language", "How To Make Lasagna", "Hot Mail"],
      answer: "Hyper Text Markup Language"
    },
    {
      question: "Which company developed JavaScript?",
      options: ["Microsoft", "Netscape", "Google"],
      answer: "Netscape"
    },
    {
      question: "What is React?",
      options: ["Library", "Game", "Browser"],
      answer: "Library"
    }
  );

  let currentIndex = 0;

  function showQuestion() {
    const current = questions[currentIndex] || {};
    const questionBox = document.getElementById("questionBox");
    const optionsBox = document.getElementById("options");

    if (!current.question) {
      questionBox.innerHTML = "🎉 Quiz Completed!";
      optionsBox.innerHTML = "";
      document.getElementById("nextBtn").style.display = "none";
      return;
    }

    questionBox.innerHTML = current.question;
    optionsBox.innerHTML = "";

    current.options.forEach((opt) => {
      const btn = document.createElement("button");
      btn.textContent = opt;

      // ✅ Using default parameter in event handler
      btn.onclick = function handleAnswer(selected = "") {
        if (selected === current.answer) {
          scoreManager.increment();
        }
        document.getElementById("score").textContent = scoreManager.getScore();
        currentIndex++;
        showQuestion();
      }.bind(null, opt); // bind selected value
      optionsBox.appendChild(btn);
    });
  }

  // Next button (works only if user skips)
  document.getElementById("nextBtn").addEventListener("click", function () {
    currentIndex++;
    showQuestion();
  });

  // Start quiz
  showQuestion();
}

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🔍 Concepts Recap

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💡 Bonus Extensions

• ✅ Add "Restart Quiz" button

• ✅ Save score in localStorage

• ✅ Show correct answer when user clicks wrong one

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🧑‍🏫 Why this project helps learners:

• Combines function concepts into a real app

• Mimics real-world quiz logic

• Can easily be extended in React later

• Strengthens modular JS thinking

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