Introduction to Classes in JavaScript

Classes in JavaScript

In the last article, we introduced some basic concepts of object-oriented programming (OOP), and discussed an example where we used OOP principles to model professors and students in a school.

We also talked about how it's possible to use prototypes and constructors to implement a model like this, and that JavaScript also provides features that map more closely to classical OOP concepts.

In this article, we'll go through these features. It's worth keeping in mind that the features described here are not a new way of combining objects: under the hood, they still use prototypes. They're just a way to make it easier to set up a prototype chain.

Prerequisites:	Basic computer literacy, a basic understanding of HTML and CSS, familiarity with JavaScript basics (see First steps and Building blocks) and OOJS basics (see Introduction to objects, Object prototypes, and Object-oriented programming).
Objective:	To understand how to use the features JavaScript provides to implement "classical" object-oriented programs.

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Source: Mozilla, https://developer.mozilla.org/en-US/docs/Learn/JavaScript/Objects/Classes_in_JavaScript#constructors_and_object_instances
This work is licensed under a Creative Commons Attribution-ShareAlike 2.5 License.

Classes and constructors

You can declare a class using the class keyword. Here's a class declaration for our Person from the previous article:

class Person {
  name;

  constructor(name) {
    this.name = name;
  }

  introduceSelf() {
    console.log(`Hi! I'm ${this.name}`);
  }
}

This declares a class called Person, with:

• a name property.
• a constructor that takes a name parameter that is used to initialize the new object's name property
• an introduceSelf() method that can refer to the object's properties using this.

The name; declaration is optional: you could omit it, and the line this.name = name; in the constructor will create the name property before initializing it. However, listing properties explicitly in the class declaration might make it easier for people reading your code to see which properties are part of this class.

You could also initialize the property to a default value when you declare it, with a line like name = '';.

The constructor is defined using the constructor keyword. Just like a constructor outside a class definition, it will:

• create a new object
• bind this to the new object, so you can refer to this in your constructor code
• run the code in the constructor
• return the new object.

Given the class declaration code above, you can create and use a new Person instance like this:

const giles = new Person("Giles");

giles.introduceSelf(); // Hi! I'm Giles

Note that we call the constructor using the name of the class, Person in this example.

Omitting constructors

If you don't need to do any special initialization, you can omit the constructor, and a default constructor will be generated for you:

class Animal {
  sleep() {
    console.log("zzzzzzz");
  }
}

const spot = new Animal();

spot.sleep(); // 'zzzzzzz'

Inheritance

Given our Person class above, let's define the Professor subclass.

class Professor extends Person {
  teaches;

  constructor(name, teaches) {
    super(name);
    this.teaches = teaches;
  }

  introduceSelf() {
    console.log(
      `My name is ${this.name}, and I will be your ${this.teaches} professor.`
    );
  }

  grade(paper) {
    const grade = Math.floor(Math.random() * (5 - 1) + 1);
    console.log(grade);
  }
}

We use the extends keyword to say that this class inherits from another class.

The Professor class adds a new property teaches, so we declare that.

Since we want to set teaches when a new Professor is created, we define a constructor, which takes the name and teaches as arguments. The first thing this constructor does is call the superclass constructor using super(), passing up the name parameter. The superclass constructor takes care of setting name. After that, the Professor constructor sets the teaches property.

Note: If a subclass has any of its own initialization to do, it must first call the superclass constructor using super(), passing up any parameters that the superclass constructor is expecting.

We've also overridden the introduceSelf() method from the superclass, and added a new method grade(), to grade a paper (our professor isn't very good, and just assigns random grades to papers).

With this declaration we can now create and use professors:

const walsh = new Professor("Walsh", "Psychology");
walsh.introduceSelf(); // 'My name is Walsh, and I will be your Psychology professor'

walsh.grade("my paper"); // some random grade

Encapsulation

Finally, let's see how to implement encapsulation in JavaScript. In the last article we discussed how we would like to make the year property of Student private, so we could change the rules about archery classes without breaking any code that uses the Student class.

Here's a declaration of the Student class that does just that:

class Student extends Person {
  #year;

  constructor(name, year) {
    super(name);
    this.#year = year;
  }

  introduceSelf() {
    console.log(`Hi! I'm ${this.name}, and I'm in year ${this.#year}.`);
  }

  canStudyArchery() {
    return this.#year > 1;
  }
}

In this class declaration, #year is a private data property. We can construct a Student object, and it can use #year internally, but if code outside the object tries to access #year the browser throws an error:

const summers = new Student("Summers", 2);

summers.introduceSelf(); // Hi! I'm Summers, and I'm in year 2.
summers.canStudyArchery(); // true

summers.#year; // SyntaxError

Note: Code run in the Chrome console can access private properties outside the class. This is a DevTools-only relaxation of the JavaScript syntax restriction.

Private data properties must be declared in the class declaration, and their names start with #.

Private methods

You can have private methods as well as private data properties. Just like private data properties, their names start with #, and they can only be called by the object's own methods:

class Example {
  somePublicMethod() {
    this.#somePrivateMethod();
  }

  #somePrivateMethod() {
    console.log("You called me?");
  }
}

const myExample = new Example();

myExample.somePublicMethod(); // 'You called me?'

myExample.#somePrivateMethod(); // SyntaxError

Summary

In this article, we've gone through the main tools available in JavaScript for writing object-oriented programs. We haven't covered everything here, but this should be enough to get you started.