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Learn how to share data and functions between objects.

Classes

Now that we know how to create an object that inherits properties from a parent class let's turn our attention to methods.

When we call `extends` in a class declaration, all of the parent methods are available to the child class. 

Below, we extend our `Animal` class to a `Cat` subclass.
```js
class Animal {
  constructor(name) {
    this._name = name;
    this._behavior = 0;
  }
    
  get name() {
    return this._name;
  }
  
  get behavior() {
    return this._behavior;
  }
  
  incrementBehavior() {
    this._behavior++;
  }
} 


class Cat extends Animal {
  constructor(name, usesLitter) {
    super(name);
    this._usesLitter = usesLitter;
  }
}

const bryceCat = new Cat('Bryce', false);
```
In the example above, our `Cat` class extends `Animal`. As a result, the `Cat` class has access to the `Animal` getters and the `.incrementBehavior()` method. 

Also in the code above, we create a `Cat` instance named `bryceCat`. Because `bryceCat` has access to the `name` getter, the code below logs `'Bryce'` to the console. 
```js
console.log(bryceCat.name);
```
Since the `extends` keyword brings all of the parent's getters and methods into the child class, `bryceCat.name` accesses the `name` getter and returns the value saved to the `name` property.

Now consider a more involved example and try to answer the following question: What will the code below log to the console?
```js
bryceCat.incrementBehavior(); // Call .incrementBehavior() on Cat instance 
console.log(bryceCat.behavior); // Log value saved to behavior
```
The correct answer is `1`. But why?
- The `Cat` class inherits the `_behavior` property, `behavior` getter, and the `.incrementBehavior()` method from the `Animal` class. 
- When we created the `bryceCat` instance, the `Animal` constructor set the `_behavior` property to zero. 
- The first line of code calls the inherited `.incrementBehavior()` method, which increases the `bryceCat` `_behavior` value from zero to one.
- The second line of code calls the `behavior` getter and logs the value saved to `_behavior` (`1`).


Inheritance IV

In the last exercise, you created a class called `Dog`, and used it to produce a `Dog` object. 

Although you may see similarities between class and object syntax, there is one important method that sets them apart. It's called the _constructor_ method. JavaScript calls the `constructor()` method every time it creates a new _instance_ of a class.

```js
class Dog {
  constructor(name) {
    this.name = name;
    this.behavior = 0;
  }
}
```
- `Dog` is the name of our class. By convention, we capitalize and PascalCase class names.
- JavaScript will invoke the `constructor()` method every time we create a new instance of our `Dog` class. 
- This `constructor()` method accepts one argument, `name`. 
- Inside of the `constructor()` method, we use the `this` keyword. In the context of a class, `this` refers to an instance of that class. In the `Dog` class, we use `this` to set the value of the Dog instance's `name` property to the `name` argument. 
- Under `this.name`, we create a property called `behavior`, which will keep track of the number of times a dog misbehaves. The `behavior` property is always initialized to zero. 

In the next exercise, you will learn how to create `Dog` instances.

Constructor

Way to go! Let's review what you learned. 

- _Classes_ are templates for objects.
- Javascript calls a _constructor_ method when we create a new instance of a class.
- _Inheritance_ is when we create a parent class with properties and methods that we can extend to child classes.
- We use the `extends` keyword to create a subclass.
- The `super` keyword calls the `constructor()` of a parent class.
- Static methods are called on the class, but not on instances of the class.

In completing this lesson, you've taken one step closer to writing efficient, production-level JavaScript. Good luck as you continue to develop your skills and move into intermediate-level concepts.


Review: Classes

At this point, we have a `Dog` class that spins up objects with `name` and `behavior` properties. Below, we will add getters and a method to bring our class to life. 

Class method and getter syntax is the same as it is for objects **except you can not include commas between methods**.
```js
class Dog {
  constructor(name) {
    this._name = name;
    this._behavior = 0;
  }
    
  get name() {
    return this._name;
  }
  
  get behavior() {
    return this._behavior;
  }
  
  incrementBehavior() {
    this._behavior++;
  }
}
```
In the example above, we add getter methods for `name` and `behavior`. Notice, we also prepended our property names with underscores (`_name` and `_behavior`), which indicate these properties should not be accessed directly. Under the getters, we add a method named `.incrementBehavior()`. When you call `.incrementBehavior()` on a Dog instance, it adds `1` to the `_behavior` property. Between each of our methods, we did _not_ include commas.


Methods

diagram: child classes Dog and Cat inherit from parent class Animal

Imagine our doggy daycare is so successful that we decide to expand the business and open a kitty daycare.  Before the daycare opens, we need to create a `Cat` class so we can quickly generate `Cat` instances. We know that the properties in our `Cat` class (`name`, `behavior`) are similar to the properties in our `Dog` class, though, there will be some differences, because of course, cats are not dogs. 

Let's say that our `Cat` class looks like this:

```js
class Cat {
  constructor(name, usesLitter) {
    this._name = name;
    this._usesLitter = usesLitter;
    this._behavior = 0;
  }
    
  get name() {
    return this._name;
  }
 
  get usesLitter() {
    return this._usesLitter;
  }

  get behavior() {
    return this._behavior;
  }  
  
  incrementBehavior() {
    this._behavior++;
  }
}
```
In the example above, we create a `Cat` class. It shares a couple of properties (`_name` and `_behavior`) and a method (`.incrementBehavior()`) with the `Dog` class from earlier exercises. The `Cat` class also contains one additional property (`_usesLitter`), that holds a boolean value to indicate whether a cat can use their litter box.

When multiple classes share properties or methods, they become candidates for _inheritance_ &mdash; a tool developers use to decrease the amount of code they need to write. 

With inheritance, you can create a _parent_ class (also known as a superclass) with properties and methods that multiple _child_ classes (also known as subclasses) share. The child classes inherit the properties and methods from their parent class.  

Let's abstract the shared properties and methods from our `Cat` and `Dog` classes into a parent class called `Animal`. 

```js
class Animal {
  constructor(name) {
    this._name = name;
    this._behavior = 0;
  }
    
  get name() {
    return this._name;
  }
  
  get behavior() {
    return this._behavior;
  }   
    
  incrementBehavior() {
    this._behavior++;
  }
} 
```
In the example above, the `Animal` class contains the properties and methods that the `Cat` and `Dog` classes share (`name`, `behavior`, `.incrementBehavior()`). 

The diagram to the right shows the relationships we want to create between the Animal, Cat, and Dog classes.

Inheritance I

Sometimes you will want a class to have methods that aren't available in individual instances, but that you can call directly from the class. 

Take the `Date` class, for example &mdash; you can both create `Date` instances to represent whatever date you want, and call _static_ methods, like `Date.now()` which returns the current date, directly from the class. The `.now()` method is static, so you can call it directly from the class, but not from an instance of the class. 

Let's see how to use the `static` keyword to create a static method called `generateName` method in our `Animal` class:
```js
class Animal {
  constructor(name) {
    this._name = name;
    this._behavior = 0;
  }
    
  static generateName() {
    const names = ['Angel', 'Spike', 'Buffy', 'Willow', 'Tara'];
    const randomNumber = Math.floor(Math.random()*5);
    return names[randomNumber];
  }
} 
```
In the example above, we create a `static` method called `.generateName()` that returns a random name when it's called. Because of the `static` keyword, we can only access `.generateName()` by appending it to the `Animal` class.

We call the `.generateName()` method with the following syntax:
```js
console.log(Animal.generateName()); // returns a name
```
You cannot access the `.generateName()` method from instances of the `Animal` class or instances of its subclasses (See below).
```js
const tyson = new Animal('Tyson'); 
tyson.generateName(); // TypeError
```
The example above will result in an error, because you cannot call static methods (`.generateName()`) on an instance (`tyson`). 


Static Methods

JavaScript is an _object-oriented programming_ (OOP) language we can use to model real-world items. In this lesson, you will learn how to make _classes_. Classes are a tool that developers use to quickly produce similar objects. 

Take, for example, an object representing a dog named `halley`. This dog's `name` (a key) is `"Halley"` (a value) and has a `behavior` (another key) of `0` (another value). We create the `halley` object below:

```js
let halley = {
  _name: 'Halley',
  _behavior: 0,
  
  get name() {
    return this._name;
  },
  
  get behavior() {
    return this._behavior;
  },
    
  incrementBehavior() {
    this._behavior++;
  }
}
```

Now, imagine you own a dog daycare and want to create a catalog of all the dogs who belong to the daycare. Instead of using the syntax above for every dog that joins the daycare, we can create a `Dog` class that serves as a template for creating new `Dog` objects. For each new dog, you can provide a value for their name. 

As you can see, classes are a great way to reduce duplicate code and debugging time.

After we lay the foundation for classes in the first few exercises, we will introduce inheritance and static methods &mdash; two features that will make your code more efficient and meaningful.



Introduction to Classes

In addition to the inherited features, child classes can contain their own properties, getters, setters, and methods. 

Below, we will add a `usesLitter` getter. The syntax for creating getters, setters, and methods is the same as it is in any other class.

```js
class Cat extends Animal {
  constructor(name, usesLitter) {
    super(name);
    this._usesLitter = usesLitter;
  }
    
  get usesLitter() {
    return this._usesLitter;
  }
}
```
In the example above, we create a `usesLitter` getter in the `Cat` class that returns the value saved to `_usesLitter`.

Compare the `Cat` class above to the one we created without inheritance:
```js
class Cat {
  constructor(name, usesLitter) {
    this._name = name;
    this._usesLitter = usesLitter;
    this._behavior = 0;
  }
    
  get name() {
    return this._name;
  }
  
  get usesLitter() {
    return this._usesLitter;
  }
  
  get behavior() {
    return this._behavior;
  }   
  
  incrementBehavior() {
    this._behavior++;
  }
}
```
We decreased the number of lines required to create the `Cat` class by about half. Yes, it did require an extra class (`Animal`), making the reduction in the size of our `Cat` class seem moot. However, the benefits (time saved, readability, efficiency) of inheritance grow as the number and size of your subclasses increase. 

One benefit is that when you need to change a method or property that multiple classes share, you can change the parent class, instead of each subclass. 

Before we move past inheritance, take a moment to see how we would create an additional subclass, called `Dog`. 

```js
class Dog extends Animal {
  constructor(name) {
    super(name);
  }
}
```
This `Dog` class has access to the same properties, getters, setters, and methods as the `Dog` class we made without inheritance, and is a quarter the size.

Now that we've abstracted animal daycare features, it's easy to see how you can extend `Animal` to support other classes, like `Rabbit`, `Bird` or even `Snake`. 


Inheritance V

In the last exercise, we created a parent class named `Animal` for two child classes named `Cat` and `Dog`. 

The `Animal` class below contains the shared properties and methods of `Cat` and `Dog`.
```js
class Animal {
  constructor(name) {
    this._name = name;
    this._behavior = 0;
  }
    
  get name() {
    return this._name;
  }
  
  get behavior() {
    return this._behavior;
  }   
  
  incrementBehavior() {
    this._behavior++;
  }
} 
```

The code below shows the `Cat` class that will inherit information from the `Animal` class.
```js
class Cat {
  constructor(name, usesLitter) {
    this._name = name;
    this._usesLitter = usesLitter;
    this._behavior = 0;
  }
    
  get name() {
    return this._name;
  }
  
  get behavior() {
    return this._behavior;
  }
  
  get usesLitter() {
    return this._usesLitter;
  }
  
  incrementBehavior() {
    this._behavior++;
  }
}
```

To the right, in **main.js**, you will put what you learned to practice by creating a parent class named `HospitalEmployee`.

Inheritance II

We've abstracted the shared properties and methods of our `Cat` and `Dog` classes into a parent class called `Animal` (See below). 
```js
class Animal {
  constructor(name) {
    this._name = name;
    this._behavior = 0;
  }
    
  get name() {
    return this._name;
  }
  
  get behavior() {
    return this._behavior;
  }
    
  incrementBehavior() {
    this._behavior++;
  }
} 
```

Now that we have these shared properties and methods in the parent `Animal` class, we can extend them to the subclass, `Cat`.

```js
class Cat extends Animal {
  constructor(name, usesLitter) {
    super(name);
    this._usesLitter = usesLitter;
  }
}
```
In the example above, we create a new class named `Cat` that extends the `Animal` class. Let's pay special attention to our new keywords: `extends` and `super`.
- The `extends` keyword makes the methods of the animal class available inside the cat class.
- The constructor, called when you create a new `Cat` object, accepts two arguments, `name` and `usesLitter`. 
- The `super` keyword calls the constructor of the parent class. In this case, `super(name)` passes the name argument of the `Cat` class to the constructor of the `Animal` class. When the `Animal` constructor runs, it sets `this._name = name;` for new `Cat` instances.
- `_usesLitter` is a new property that is unique to the `Cat` class, so we set it in the `Cat` constructor.

Notice, we call `super` on the first line of our `constructor()`, then set the `usesLitter` property on the second line. In a `constructor()`, you must always call the `super` method before you can use the `this` keyword &mdash; if you do not, JavaScript will throw a reference error. To avoid reference errors, it is best practice to call `super` on the first line of subclass constructors.
 
Below, we create a new `Cat` instance and call its name with the same syntax as we did with the `Dog` class:
```js
const bryceCat = new Cat('Bryce', false); 
console.log(bryceCat._name); // output: Bryce
```
In the example above, we create a new instance the `Cat` class, named `bryceCat`. We pass it `'Bryce'` and `false` for our `name` and `usesLitter` arguments. When we call `console.log(bryceCat._name)` our program prints, `Bryce`.

In the example above, we abandoned best practices by calling our `_name` property directly. In the next exercise, we'll address this by calling an inherited getter method for our `name` property.

Inheritance III

Finally, let's use our new methods to access and manipulate data from `Dog` instances. 
```js
class Dog {
  constructor(name) {
    this._name = name;
    this._behavior = 0;
  }
    
  get name() {
    return this._name;
  }
  
  get behavior() {
    return this._behavior;
  }   
  
  incrementBehavior() {
    this._behavior++;
  }
}

const halley = new Dog('Halley');
```
In the example above, we create the `Dog` class, then create an instance, and save it to a variable named `halley`. 

The syntax for calling methods and getters on an instance is the same as calling them on an object &mdash; append the instance with a period, then the property or method name. For methods, you must also include opening and closing parentheses.

  Let's take a moment to create two `Dog` instances and call our `.incrementBehavior()` method on one of them.

```js
let nikko = new Dog('Nikko'); // Create dog named Nikko
nikko.incrementBehavior(); // Add 1 to nikko instance's behavior
let bradford = new Dog('Bradford'); // Create dog name Bradford
console.log(nikko.behavior); // Logs 1 to the console
console.log(bradford.behavior); // Logs 0 to the console
```
In the example above, we create two new `Dog` instances, `nikko` and `bradford`. Because we increment the behavior of our `nikko` instance, but not `bradford`, accessing `nikko.behavior` returns `1` and accessing `bradford.behavior` returns `0`.



Method Calls

Now, we're ready to create class instances. An _instance_ is an object that contains the property names and methods of a class, but with unique property values. Let's look at our `Dog` class example.

```js
class Dog {
  constructor(name) {
    this.name = name;
    this.behavior = 0;
  } 
}

const halley = new Dog('Halley'); // Create new Dog instance
console.log(halley.name); // Log the name value saved to halley
// Output: 'Halley'
```
Below our `Dog` class, we use the `new` keyword to create an instance of our `Dog` class. Let's consider the line of code step-by-step.
- We create a new variable named `halley` that will store an instance of our `Dog` class.
- We use the `new` keyword to generate a new instance of the `Dog` class. The `new` keyword calls the `constructor()`, runs the code inside of it, and then returns the new instance.
- We pass the `'Halley'` string to the `Dog` constructor, which sets the `name` property to `'Halley'`.
- Finally, we log the value saved to the `name` key in our `halley` object, which logs `'Halley'` to the console.

Now you know how to create instances. In the next exercise, you will learn how to add getters, setters, and methods.

