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

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


// 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 function is executed immediately!');
})();

// 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); // Correct sort for numbers
console.log(arr); // Outputs [1, 1, 3, 4, 5, 9]

# Advanced Features of Functions

Let's dive into more advanced techniques for using functions.

<br />

## Callback Functions

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

```javascript title="Invoking a Passed Callback Function"
function study(callback) {
  console.log('Studying...');
  callback(); // call the callback function
}

function call() {
  console.log('Callback function called!');
}

study(call);
```

In the above code, the `study` function takes the `call` function as an argument and executes it.

Here, the `call` function is the callback function of the `study` function.

When executing `study(call)`, the `call` function is invoked, outputting `Studying...` and `Callback function called!`.

<br />

## Recursive Function

A recursive function is a function that calls itself.

```javascript title="Implementing Factorial Using 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)

An IIFE is a pattern for defining and immediately invoking a function.

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

<br />

## Built-in JavaScript Functions

JavaScript provides several useful built-in functions, such as those for sorting arrays or slicing strings.

Functions provided by JavaScript itself are called `built-in functions`.

For example, operations like sorting an array or slicing a string are supported by basic functions.

```javascript title="String Slicing"
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 called at a specific point within that function. This is useful for asynchronous operations or event handling.

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