🧠 10-Day Challenge: Day 5 Functions & Scope in JS

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📅 Day 5: Functions & Scope

Welcome to Day 5 of the challenge!4

Today, we’re unlocking the core building blocks of JavaScript: Functions and Scope. Functions help you organize, reuse, and simplify your code — and understanding scope gives you control over your variables.

🔧 What Is a Function?
A function is a reusable block of code that performs a specific task.
You define it once and use it wherever you need it.

Think of a function like a vending machine:

Insert input → Process → Get output

✅ Function Declaration

function greet(name) {
  return "Hello, " + name + "!";
}
console.log(greet("Smriti")); // Hello, Smriti!

✅ Function Expression

const greet = function(name) {
  return "Hi, " + name + "!";
};

Function expressions are not hoisted, so you can't call them before they’re defined.

⚡ Arrow Function (ES6+)
Arrow functions provide a shorter syntax and behave differently with this.

const greet = (name) => {
  return "Hey, " + name + "!";
};

If the function has only one expression, you can make it even shorter:

const greet = name => "Hey, " + name + "!";

🔄 Parameters vs Arguments
Parameters: Variables listed in the function definition
function greet(name) → name is a parameter

Arguments: Actual values passed when calling the function
greet("Smriti") → "Smriti" is an argument

🔁 The return Keyword
The return keyword is used to output a value from a function.

function add(a, b) {
  return a + b;
}
console.log(add(3, 4)); // 7

If no return, the function returns undefined.

📦 Scope in JavaScript

Scope determines where your variables can be accessed.

✅ Function Scope
Variables declared with var are function-scoped:

function showMessage() {
  var message = "Hello";
  console.log(message);
}
// message is not accessible outside

✅ Block Scope
let and const are block-scoped:

if (true) {
  let msg = "Inside block";
  const x = 42;
}
// msg and x are not accessible here

🔄 Normal Function vs Arrow Function: What's the Difference?

✅ Mini Task: Factorial Function

Write a function that calculates the factorial of a number.

💡 Factorial of 5 \= 5 × 4 × 3 × 2 × 1 = 120

💻 Solution:

function factorial(n) {
  let result = 1;
  for (let i = 1; i <= n; i++) {
    result *= i;
  }
  return result;
}

console.log(factorial(5)); // 120

Try writing the same using a recursive function!

❓ Interview Questions (Day 5 Topics)

1. What’s the difference between a function declaration and a function expression?

2. Explain the difference between parameters and arguments.

3. What does the return keyword do in a function?

4. What is the difference between function scope and block scope?

5. What are the key differences between arrow functions and regular functions?

�� That’s a Wrap for Day 5!
Now you understand how to create and use functions, pass and return values, and manage variable visibility through scope.
Functions = power + reusability.

👉 Coming up in Day 6: Arrays & Array Methods—where we’ll learn how to store and work with lists of data.

Keep coding, and share your factorial function below, which you wrote with a different logic! 💪💻