Adding methods to the class

Hi Richi,

To start with, let's make this code readable! (I also removed a line that you commented out)

class Pet 
{ 
        constructor(animal, age, breed, sound)
        {
                this.animal = animal;
                this.age = age;
                this.breed = breed;
                this.sound = sound; 
        } 

        //Adding methods to the class         
        speak()
        { 
                console.log(this.sound); 
        } 
}

const ernie = new Pet('dog', 1, 'pug', 'yip yip'); 
const vera = new Pet('dog', 6, 'border collie', 'woof woof');

ernie.speak(); vera.speak();

Phew, that's nicer. Now I can see what's going on.

Okay, so what kind of scope does the sound parameter have? This is a really good question and its great that you're thinking about how classes work.

Let's change the syntax a bit to avoid confusion. Let's say we're working with this.sound = argument_four. Here, sound is a property of Pet and its value is being set to the argument passed to the constructor. In short, sound is scoped to the Pet object created by the class.

That's scope, but your question hints other aspects of object oriented programming, privacy and inheritence. In JavaScript (unlike other OOP languages such as Java) class properties are all public! This means I could simply write console.log(vera.sound) from anywhere that has vera in scope.

There is a (complicated, advanced) way to make JavaScript class properties private, but it requires other tools and code build steps to essentially extend the JavaScript language.

If we were coding in Java instead we would be able to make the class properties one of public, private, or protected. Here, public means they would work the same as JavaScript - they can be accessed however you like. Private means they can only be used by that class.

You might find it interesting that even if this.sound was private the speak() method could still access it, because speak() is a method of class Pet, so it can access Pet's private properties and methods. It would only be other functions and classes that couldn't access Pet's private properties.

Protected means that it's private, unless a subclass wants to access it. Don't worry too much about this (or any of this part about Java really).

I hope that answers your question and isn't an information overload. Let me know if you need any more info :)

Cheers,

Eric

Many thanks Eric, I'm very sorry for the format of my code. Once I copied and paste it it totally changes... How can I make the code shows in the format you showed us? inside the black box. Thanks!