Object-Oriented Programming {

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Summary

Object-Oriented Programming (OOP) allows us to define classes of objects with specific properties and behaviours. We can then create multiple objects (instances) out of these classes, leading to modularity and reuse of code. With inheritance we can base one class on another class, leading to further reuse.

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Contents

• Defining a class
• Creating objects from a class
• Inheritance

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Defining a class

We use the class syntax to define classes, including their name, their constructor, their properties, and their methods…

class Rabbit {
  constructor(name, age, cuteness) {
    this.name = name;
    this.age = age;
    this.cuteness = cuteness;
  }

  sayHi() {
    console.log(`Hello, my name is ${this.name}!`);
  }
}

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Creating objects from a class

We create objects from classes using the special word new and their constructor function (which has the same name as the class)

let miffy = new Rabbit("Miffy", 63, 100);
let jiffy = new Rabbit("Jiffy", 12, 20);

We use these objects with the same dot notation we already saw for object literals

miffy.sayHi(); // "Hello, my name is Miffy!"
jiffy.sayHi(); // "Hello, my name is Jiffy!"

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Inheritance

We can create classes based on other classes using inheritance:

class Animal {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }

  sayHi() {
    console.log(`Hi, my name is ${this.name}!`);
  }
}

To create a class based on the Animal class, we use the extends keyword:

class Rabbit extends Animal {
  constructor(name, age, cuteness) {
    super(name, age); // Calls the parent's (Animal's) constructor
    this.cuteness = cuteness; // A new property just for a Rabbit!
    // Our Rabbit already has name and age properties thanks to being an Animal
    // so we don't need to deal with them here
  }

  // Our Rabbit already has the sayHi() method thanks to being an Animal
  // so we don't need to define it here

  isCute() { // A new method just for a Rabbit
    return (this.cuteness > 50); // true if cuteness is greater than 50
  }
}

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Using classes with inheritance

A class that extends another class has all the properties and methods of the parent class as well as anything the child class adds on:

let miffy = new Rabbit("Miffy", 63, 100); // Creates a Rabbit object

miffy.sayHi(); // "Hi, my name is Miffy!" because it uses the Animal properties and sayHi() method

// We can also use the Rabbit's new isCute() method and cuteness property:
if (miffy.isCute()) { // true because we set miffy's cuteness to 100
  console.log("Miffy is cute!"); // "Miffy is cute!"
}

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}