🔹 What is a Function?

A function is a block of code that performs a specific task and can be reused.

👉 Instead of writing code again and again, you call a function.

function greet() {
  console.log("Hello!");
}
greet(); // calling function

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🔹 Function Declaration

Defined using the function keyword.

function add(a, b) {
  return a + b;
}console.log(add(5, 3)); // 8

✔️ Can be called before declaration (hoisting)

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🔹 Function Expression

Function stored inside a variable.

const multiply = function(a, b) {
  return a * b;
};console.log(multiply(4, 3)); // 12

❌ Cannot call before definition

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🔹 Arrow Functions (ES6)

Short and modern syntax.

const divide = (a, b) => {
  return a / b;
};console.log(divide(10, 2)); // 5

👉 Short form:

const square = x => x * x;

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🔹 Parameters & Arguments

• Parameters → variables in function
• Arguments → actual values passed

function greet(name) { // parameter
  console.log("Hello " + name);
}greet("Aditya"); // argument

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🔹 Return Values

Functions can return a value.

function sum(a, b) {
  return a + b;
}let result = sum(2, 3);
console.log(result); // 5

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🔹 Default Parameters

Set default value if no argument is passed.

function greet(name = "Guest") {
  console.log("Hello " + name);
}greet(); // Hello Guest
greet("Aditya"); // Hello Aditya

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🔹 Callback Functions

A function passed as an argument to another function.

function greet(name, callback) {
  console.log("Hello " + name);
  callback();
}function sayBye() {
  console.log("Goodbye!");
}greet("Aditya", sayBye);

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🧪 Practice Programs

1️⃣ Age Calculator

function calculateAge(birthYear) {
  let currentYear = new Date().getFullYear();
  return currentYear - birthYear;
}let age = calculateAge(2000);
console.log("Your age is:", age);

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2️⃣ Simple Interest Calculator

Formula:
👉 SI = (P × R × T) / 100

function simpleInterest(p, r, t) {
  return (p * r * t) / 100;
}let si = simpleInterest(10000, 5, 2);
console.log("Simple Interest:", si);