JavaScript

📌 What is a Function?

A function is a block of code designed to perform a task. You can reuse this block anywhere in your code, saving time and avoiding repetition.

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✅ Why Use Functions?

• Code reusability
• Better organization
• Easier debugging and maintenance
• Avoid repetition (DRY – Don’t Repeat Yourself)

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🛠️ 1. Function Declaration (Named Function)

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

greet(); // Call the function

• The function keyword defines a function.
• greet is the function name.
• () holds parameters (if any).
• {} contains the function code (body).

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🧾 2. Function with Parameters

function greetUser(name) {
  console.log("Hello, " + name);
}

greetUser("Alice");
greetUser("Bob");

• name is a parameter.
• "Alice" and "Bob" are arguments passed to the function.

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🔙 3. Function with Return Value

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

let result = add(5, 3);
console.log(result); // Output: 8

• return sends a value back to the place where the function was called.

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🧑‍💻 4. Function Expressions

const multiply = function(x, y) {
  return x * y;
};

console.log(multiply(4, 5)); // Output: 20

• Functions can be stored in variables.
• These are anonymous functions (no name).

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⚡ 5. Arrow Functions (ES6+)

const divide = (a, b) => {
  return a / b;
};

console.log(divide(10, 2)); // Output: 5

• Shorter syntax using =>
• If there’s only one statement, you can write it even shorter:

const square = n => n * n;
console.log(square(4)); // Output: 16

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♻️ 6. Default Parameters

function greet(name = "Guest") {
  console.log("Welcome, " + name);
}

greet();         // Welcome, Guest
greet("Sam");    // Welcome, Sam

• If no argument is passed, default is used.

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📦 7. Rest Parameters (...)

function sum(...numbers) {
  let total = 0;
  for (let num of numbers) {
    total += num;
  }
  return total;
}

console.log(sum(1, 2, 3, 4)); // Output: 10

• Collects all extra arguments into an array-like object.

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🔍 8. Function Scope

function testScope() {
  let a = 10; // local to function
  console.log(a);
}

testScope();
// console.log(a); ❌ Error: a is not defined

• Variables declared inside a function are local to that function.

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🧠 Summary

Type	Syntax Example
Function Declaration	function greet() {}
Function Expression	const greet = function() {}
Arrow Function	const greet = () => {}
Return Value	return something;
Parameters/Arguments	function greet(name) {}

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