Classes

Follow along with code examples here!

• Intro

• Factory Functions Waste Memory

• Classes

  • Class Definition and new

  • Instanceof

  • Setting Properties With A Constructor

  • Defining Instance Methods

• Quiz!

• Challenge

• Summary

Intro

In the last lecture, we learned about encapsulation - bundling data and methods that act on that data into an object. We learned about using closures to create private variables.

const makeFriendsManager = (...initialFriends) => {
  const friends = [...initialFriends];

  return {
    getFriends() {
      return [...friends]; 
    },
    addFriend(newFriend) {
      if (typeof newFriend !== 'string') return;
      friends.push(newFriend);
    }
  }
}

Factory Functions Waste Memory

The nice thing about encapsulation is that we can re-use makeFriendsManager to create multiple objects that look alike: each friends manager has getFriends and addFriends methods.

This kind of function is called a factory function and each object created from this factory function is called an instance.

// factory functions return objects
const makeFriendsManager = (...initialFriends) => {
  const friends = [...initialFriends];

  return {
    getFriends() {...},
    addFriend(newFriend) { ... },
  }
}

// instances of the factory function
const myFM = makeFriendsManager();
const yourFM = makeFriendsManager();

The objects myFM and yourFM definitely have the same behavior. But do they share that behavior? That is, are the methods they each have referencing the same exact function?

// are these the same object?
console.log(myFM === yourFM)

// are the methods of these objects the same?
console.log(myFM.addFriend === yourFM.addFriend)

Q: Are the methods myFM.addFriend() and yourFM.addFriend() referencing the same exact function?

No! They are not the same. Each time the factory function is invoked, a brand new object is made and the methods are recreated as well. This is a waste of memory.

Classes

classes define properties and methods that instances inherit

A class defines a type of object and the properties/methods that those objects will share.

Q: Suppose we wanted to create a class to represent users. What would the default properties be? What methods would be shared by each instance?

• The User class would have a constructor function for making a User instance with properties like username, email, and password

• The User class might have methods like changeUsername or setPassword

Class Definition and new

Many programming languages implement classes in some manner.

In JavaScript, it starts with the class keyword, an uppercase name, and curly braces. Like this:

// class definitions
class User {

}

class Pet {

}

// creating class instances
const ben = new User();
const clifford = new Pet();

// Instances are objects derived from a particular class
console.log(ben); // User {}
console.log(clifford); // Pet {}

With a class definition, we can create new instances of that class using the new keyword. An instance is an object that is derived from a class.

Note: Even though User is treated like a function (we invoke it), you must use the new keyword when making an instance (you'll get an error if you don't)

// User is a function, but you can't just call it
console.log(typeof User); // function

const ben = User(); // error: you must use the new keyword to invoke a constructor function

Instanceof

We can use the instanceof operator (kind of like the typeof operator) to see if an object is derived from the given class.

console.log(ben instanceof User); // true
console.log(ben instanceof Pet); // false

console.log(clifford instanceof User); // false
console.log(clifford instanceof Pet); // true

Setting Properties With A Constructor

Right now, the class definitions only allow us to create blank objects. But objects are only useful if they have properties.

There are two kinds of properties that instances of a class can have:

1. Properties with default values that all instances start with

2. Properties whose values are provided when the instance is made

class User {
  // Default instance properties are defined here. We can change these later.
  // Notice that the `this` keyword isn't used
  isAdmin = false;
  password = null; 

  // Instance properties that require inputs go in the constructor
  constructor(name, email) {
    this.name = name;  // <-- The `this` keyword references the new instance object being created
    this.email = email;
  }
}

const ben = new User('ben', '[e-mail]');
const zo = new User('zo', '[e-mail]');

console.log(ben, zo);
// User {isAdmin: false, password: null, name: 'ben', email: '[e-mail]'}
// User {isAdmin: false, password: null, name: 'zo', email: '[e-mail]'}

Class constructor functions have some quirks to get used to:

• constructor is a special method name. You must use this name. When you create a new instance of a class using new, JavaScript will look to see if the class has a constructor method and it will execute that method.

• The constructor function can accept parameters whose values are provided when the instance is made

• The this keyword, when used in a constructor, references the new instance object being created.

Defining Instance Methods

Remember, encapsulation wants us to bundle data with methods that operate on that data.

Adding methods to a class definition looks like this:

class User {
  isAdmin = false;
  password = null;

  constructor(name, email) {
    this.name = name;
    this.email = email;
  }
  
  // notice that we don't have commas between methods
  // These methods are shared by ALL instances of the class

  setPassword(newPassword) {
    // When used in a method, this references the object invoking the method
    this.password = newPassword;
  }

  validatePassword(passwordToCheck) {
    if (!this.password) {
      console.log('No password set.');
      return false;
    }
    if (passwordToCheck === this.password) {
      console.log('It matches!');
      return true;
    }
    console.log('Wrong password!');
    return false;
  }
}
const ben = new User('ben', '[e-mail]');

ben.validatePassword('1234'); // No password set.
ben.setPassword('1234');
ben.validatePassword('1234'); // It Matches!

When used in a method, the this keyword refers to the object invoking the method.

const ben = new User('ben', '[e-mail]');
const zo = new User('zo', '[e-mail]');

// they are the same method
console.log(ben.setPassword === zo.setPassword); // true

// when we invoke the method, the value of `this` changes
ben.setPassword('1234');
zo.validatePassword('1234'); // No password set.

Next time, we'll look at making the password private.

Quiz!

Can you spot the mistake(s) with the code below?

const Animal = {
  this.owners = [];

  constructor: (species, sound) => {
    this.species = species;
    this.sound = sound;
  },
  makeSound() {
    console.log(sound)
  }
}

const dog = Animal('canine', 'woof');

Q: Answer

The following mistakes are made:

• const is used instead of class to define the Animal class

• We don't need the = to create the class

• The owners property with the default value doesn't need this

• The constructor function should be written like this: constructor () {} without the : and =>

• We don't need a comma to separate the methods

• makeSound should use this.sound

• When creating an instance of Animal, the new keyword should be used.

class Animal {
  owners = [];

  constructor (species, sound) {
    this.species = species;
    this.sound = sound;
  }
  makeSound() {
    console.log(this.sound)
  }
}

const dog = new Animal('canine', 'woof');

Challenge

Create a class called FoodItem. Every instance of FoodItem should have the following properties and methods

• name — the name of the item

• price - the price of the item in US dollars

• weight - the weight of the item

• getPricePerPound() - returns the price / pound of the item

For example, I should be able to use this FoodItem class like so

const apple = new FoodItem('apple', 1, 0.5);
console.log(apple); 
// FoodItem { name: 'apple', price: 1, weight: 0.5 }

console.log(apple.getPricePerPound());
// 2

Q: Solution

class FoodItem {
  constructor(name, price, weight) {
    this.name = name;
    this.price = price;
    this.weight = weight;
  }
  getPricePerPound() {
    return this.price/this.weight;
  }
}

Now, create a second class called ShoppingCart. Every instance of ShoppingCart should have the following properties and methods:

• items — an array that starts empty. It should hold FoodItem instances.

• addItem(FoodItem) — takes in a FoodItem instance and adds it to the items array.

• getTotalPrice() - calculates the total price of all FoodItems in the items array

For example, I should be able to use this ShoppingCart class like so

const myCart = new ShoppingCart();
console.log(myCart); // ShoppingCart { items: [] }

myCart.addItem(new FoodItem('apple', 1, 0.5)) // name, price, weight
myCart.addItem(new FoodItem('bread', 5, 1))
myCart.addItem(new FoodItem('cheese', 7, 2))
console.log(myCart); // ShoppingCart { items: Array(3) }

console.log(myCart.getTotalPrice()); // 13

Q: Solution

class ShoppingCart {
  constructor() {
    this.items = [];
  }
  addItem(item) {
    // we can get fancy and ensure that the incoming item is a FoodItem with instanceof
    if (!(item instanceof FoodItem)) return;

    this.items.push(item);
  }
  getTotalPrice() {
    return this.items.reduce((total, item) => total + item.price, 0)
  }
}

Summary

• A class defines a type of object with shared methods and properties

  • It has a constructor function for defining the default properties that every instance of that class (objects of that type) will have.

  • All instances of that class inherit the class' methods.

• Classes are defined using the class keyword

• Instances of a class are created using the new keyword and the class constructor.

• When used in a constructor function, this points to the newly created object

  • When used in a method, this points to the object invoking the method

class Animal {
  owners = [];
  constructor (species, sound) {
    this.species = species;
    this.sound = sound;
  }
  makeSound() {
    console.log(this.sound)
  }
}

const dog = new Animal('canine', 'woof');
dog.makeSound(); // 'woof'

const cat = new Animal('feline', 'meow');
cat.makeSound(); // 'meow'