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// Callback function
function study(callback) {
  console.log('Studying...');
  callback(); // Call the callback function
}

study(() => {
  console.log('Callback function');
});

// Recursive factorial function
function factorial(n) {
  if (n === 1) return 1;
  return n * factorial(n - 1);
}
console.log(factorial(5)); // Outputs 120

// Immediately Invoked Function Expression (IIFE)
(function () {
  console.log('This is an immediately invoked function!');
})();

// Using the built-in sort function to sort an array in ascending order
let arr = [3, 1, 4, 1, 5, 9];
arr.sort((a, b) => a - b); // Correctly sort numbers numerically
console.log(arr); // Outputs [1, 1, 3, 4, 5, 9]

# Advanced Features of Functions

Let's explore more advanced ways to use functions.

<br />

## Callback Function

A callback function is a function that is passed as an argument to another function.

```javascript title="Calling a callback function passed as an argument"
function study(callback) {
  console.log('Studying...');
  callback(); // Call the callback function
}

study(() => {
  console.log('Finished studying!');
});
```

<br />

## Recursive Function

A recursive function is a function that calls itself.

```javascript title="Implementing factorial with a recursive function"
function factorial(n) {
  if (n === 1) return 1;
  return n * factorial(n - 1);
}

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

<br />

## IIFE (Immediately Invoked Function Expression)

IIFE is a pattern where a function is defined and immediately invoked.

Example:

```javascript title="Using IIFE"
(function () {
  console.log('This function runs immediately!');
})();
```

<br />

## Built-in Functions and Usage Examples

JavaScript provides many useful built-in functions. For example, it supports functions to sort arrays or slice strings.

Example:

```javascript title="Using the built-in sort function"
let arr = [3, 1, 4, 1, 5, 9];
arr.sort();
console.log(arr); // Outputs [1, 1, 3, 4, 5, 9]

let str = 'Hello, world!';
let slicedStr = str.slice(0, 5);
console.log(slicedStr); // Outputs "Hello"
```

A callback function is passed as an argument to another function and is invoked at a specific time within that function. This is useful for asynchronous tasks or event handling.

### A callback function is a function that is passed as an argument to another function.