This course is for JavaScript developers who are preparing for their interviews and want to learn about the types of design patterns. The course is divided into four sections, each explaining one category of the design pattern. The types covered in this course include creational, structural, behavioral, and architectural design patterns. Each pattern is explained with the help of examples and is followed by a challenge and its solution review.

Acing the JavaScript Design Patterns Interview

## Problem statement

In this challenge, you need to implement the visitor pattern to separate `Rock` music from the other genres of music by creating a separate playlist. You need to implement the `RockMusicVisitor` class for this purpose. As you can see, it contains the `visit` method; this should return the rock music playlist.

>**Note:** the rock music playlist only needs to contain the names of the songs, not the entire `Song` object.

Next, you need to implement a `Song` class. A `Song` object will have the following properties: `name` and `genre`. You also need to define the two functions: `getName` and `getGenre`.

Lastly, you need to implement the `MusicLibrary` class. It should store a list of all the songs regardless of the genres. You need to implement the `addSong` function to add a song and `accept` function to allow the `RockMusicVisitor` to visit the music library.  

## Input

The `accept` function is called with `RockMusicVisitor` as a parameter


## Output

A playlist containing names of all the rock songs from the library is returned.


## Sample input

```
var rockMusicVisitor = new RockMusicVisitor()
var song1 = new Song("Bohemian Rhapsody","Rock")
var song2 = new Song("Stair way to Heaven","Rock")
var song3 = new Song("Opps I did it again", "Pop")
var song4 = new Song("Crazy", "Country")
var musicLibrary = new MusicLibrary()
musicLibrary.addSong(song1)
musicLibrary.addSong(song2)
musicLibrary.addSong(song3)
musicLibrary.addSong(song4)
console.log(musicLibrary.accept(rockMusicVisitor))
```
## Sample output

```
["Bohemian Rhapsody","Stair way to Heaven"]
```

## Challenge 
Take a close look and design a step-by-step solution before jumping on to the implementation. This problem is designed for your practice, so try to solve it on your own first. If you get stuck, you can always refer to the solution provided. Good Luck!

class RockMusicVisitor{
    //write your code here
    visit(){}
}

class Song{
   //write your code here 
   getName(){}
   getGenre(){}
}

class MusicLibrary{
   //write your code here 
   addSong(){}
   accept(){}
}

class RockMusicVisitor{
    visit(musicLibrary) {
        var rockPlayList=[]; 
        var index = 0;
        var songlist = musicLibrary.songsList.length 
        for(var i=0; i<songlist; i++){
            var song = musicLibrary.songsList[i]
            if(song.getGenre() == "Rock"){
                rockPlayList[index] = song.getName()
                index++
            }
        }
        return rockPlayList
    }
}

class Song{
    constructor(name,genre){
        this.name = name
        this.genre = genre
    }

    getName(){
        return this.name
    }
    getGenre(){
        return this.genre
    }
}

class MusicLibrary{
    constructor(){
        this.songsList = []
    }
    addSong(song){
        this.songsList.push(song)
    }
    accept(visitor){
        return visitor.visit(this)
    }
}

-----
Let’s discuss the solution in the next lesson.

# Problem statement

In this challenge, you need to implement the visitor pattern to separate `Rock` music from the other genres of music by creating a separate playlist. You need to implement the `RockMusicVisitor` class for this purpose. As you can see, it contains the `visit` method; this should return the rock music playlist.

>**Note:** the rock music playlist only needs to contain the names of the songs, not the entire `Song` object.

Next, you need to implement a `Song` class. A `Song` object will have the following properties: `name` and `genre`. You also need to define the two functions: `getName` and `getGenre`.

Lastly, you need to implement the `MusicLibrary` class. It should store a list of all the songs regardless of the genres. You need to implement the `addSong` function to add a song and `accept` function to allow the `RockMusicVisitor` to visit the music library.  

# Input

The `accept` function is called with `RockMusicVisitor` as a parameter


# Output

A playlist containing names of all the rock songs from the library is returned.


# Sample input

```
var rockMusicVisitor = new RockMusicVisitor()
var song1 = new Song("Bohemian Rhapsody","Rock")
var song2 = new Song("Stair way to Heaven","Rock")
var song3 = new Song("Opps I did it again", "Pop")
var song4 = new Song("Crazy", "Country")
var musicLibrary = new MusicLibrary()
musicLibrary.addSong(song1)
musicLibrary.addSong(song2)
musicLibrary.addSong(song3)
musicLibrary.addSong(song4)
console.log(musicLibrary.accept(rockMusicVisitor))
```
# Sample output

```
["Bohemian Rhapsody","Stair way to Heaven"]
```

# Challenge 
Take a close look and design a step-by-step solution before jumping on to the implementation. This problem is designed for your practice, so try to solve it on your own first. If you get stuck, you can always refer to the solution provided. Good Luck!

In this challenge, you have to implement the visitor pattern to solve the given problem.

Challenge: Visitor Pattern

Problem statement

Input

Output

Sample input

Sample output

Challenge #