A deep dive into ES6 Classes

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~ 6 min read
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javascript
Author

Mustapha Aouas
A deep dive into ES6 Classes

Classes were introduced in ECMAScript 6, and we can use them to structure our code in a traditional OOP fashion by defining a template for creating objects. In this post we'll learn everything about ES6 classes, then we will compare them to constructor functions and prototypal inheritance.

A quick word before we start. This article is intended to appeal to a wide range of readers. So, if you're an advanced JS user, you can use the table of contents below to select which sections to read. If, on the other hand, you're just getting started with JS and you're having trouble understanding something, feel free to ask me in the comments section.

Table of Contents

 

We will see how to define classes and how to create objects using them, then we will talk about inheritance and more - But first, let's start right away by taking a look at the anatomy of a class.

Anatomy of a class

The class keyword

To declare a class we use the class keyword followed by the name of the class.

Class declaration 

class Point {
  constructor() {}
}

In the snippet above we declared a "Point" class. This is called a class declaration.

Note that I'm using the PascalCase notation for the name of the class. This is not mandatory but a common convention.

In fact classes are special functions, and like with functions, you can use either class declarations or class expressions.

Class expression

This is a class expression:

let Point = class {
  constructor() {}
}

Constructor

The constructor method is a special method for creating and initialising an object created with a class.

There can only be one constructor in each class. A SyntaxError will be thrown if the class contains more than one occurrence of a constructor.

It is not mandatory to have a constructor in the class definition. The code bellow is valid.

class Point { }

Properties

Instance properties

Instance properties must be defined inside of class methods. In the snippet below x and y are instance properties:

class Point {
  constructor(a, b) {
    this.x = a;
    this.y = b;
  }
}

Fields

The code can be more self documenting by declaring fields up-front. Let's refactor the code above using fields, and while we're at it, let's give them a default value:

class Point {
  x = 0;
  y = 0;

  constructor(a, b) {
    this.x = a;
    this.y = b;
  }
}

Note that fields are always present whereas instance properties must be defined inside of class methods. Note also that fields can be declared with or without a default value.

Private fields

To declare a private field all you have to do is prefix its name with #. See the code below:

class Point {
  #x = 0;
  #y = 0;

  constructor(a, b) {
    this.#x = a;
    this.#y = b;
  }
}

Trying to access a private field outside the scope of the class will result in a syntax error.

Note that instance properties can not be private, only fields can. So you can't create an instance property with the # prefix. This would result in a syntax error.

Methods

Public methods

To declare a method we can use the ES6 shorter syntax for method definitions on objects:

class Point {
  #x = 0;
  #y = 0;

  translate(a, b) {
    this.#x += a;
    this.#y += b;
  }
}

Private methods

Like we did with private fields, we can use a # as a prefix of our private methods:

class Point {
  #x = 0;
  #y = 0;

  constructor(x, y) {
    this.#setXY(x, y)
  }

  translate(a, b) {
    this.#setXY(
      this.#x + a,
      this.#y + b);
  }

  // Private method
  #setXY(x, y) {
    this.#x = x;
    this.#y = y;
  }
}

Generator methods

The same way as public methods we can declare generator methods:

class Point {
  #x = 0;
  #y = 0;
  #historyPositions = [];

  translate(a, b) {
    this.#x += a;
    this.#y += b;

    this.#historyPositions.unshift(
      [this.#x, this.#y]
    );
  }

  *getHistoryPositions() {
    for(const position of this.#historyPositions){
      yield position;
    }
  }
}

In the snippet above we declared a getHistoryPositions generator method.

Note: to declare a private generator method use this syntax: *#getHistoryPositions() {}.

Getters and Setters

To implement getters and setters we use the get and set keyword:

Here is an example:

class Point {
  #x = 0;
  #y = 0;

  get position() {
    return [this.#x, this.#y];
  }

  set position(newPosition) {
    // newPosition is an array like [0, 0]
    [this.#x, this.#y] = newPosition;
  }
}

Static fields and methods

Static methods and fields (of a class) can be defined using the static keyword. Static members (fields and methods) cannot be called through a class instance and must be called without instantiating the class.

Static methods are frequently used to construct utility functions, whereas static properties are excellent for caching, fixed-configuration, or any other data that does not need to be copied across instances.

Here is an example of a static method:

class Point {
  static isEqual(pointA, pointB) {
    const [x1, y1] = pointA.position;
    const [x2, y2] = pointB.position;
    return x1 === x2 && y1 === y2;
  }

  #x = 0;
  #y = 0;

  get position() {
    return [this.#x, this.#y];
  }

  constructor(a, b) {
    [this.#x, this.#y] = [a, b];
  }
}

// Consider that p1 and p2 are both instances of Point
Point.isEqual(p1, p2) // Boolean

Creating an object with a class

The new keyword

To create a new instance of a class we use the new keyword:

class Point {}

const point = new Point();

Hoisting

Function declarations and class declarations can be distinguished by the fact that function declarations are hoisted whereas class declarations are not. You must first define and then access your class; otherwise, code like this will throw a ReferenceError:

const point = new Point(); // ReferenceError

class Point {}

Inheritance

The extends keyword

In class declarations or class expressions, the extends keyword is used to create a class that is a child of another class (a subclass). We'll look at an example in the next section.

Super

The super keyword is used to access and call functions on an object's parent. If there is a constructor present in the subclass, it needs to first call super() before using this.

See the code below:

class Vehicle {
  #numberOfPassengers = 0;

  constructor(nb) {
    this.#numberOfPassengers = nb;
  }

  getNumberOfPassengers() {
    return this.#numberOfPassengers;
  }
}

class Car extends Vehicle {
  constructor() {
    super(5);
  }
}

class Bike extends Vehicle {
  constructor() {
    super(1);
  }
}

const car = new Car();
const bike = new Bike();

car.getNumberOfPassengers(); // 5
bike.getNumberOfPassengers(); // 1

Metadata

In class constructors, new.target refers to the constructor that was called directly by new. This is also true if the constructor belongs to a parent class and was delegated from a child constructor.

class Vehicle {
  constructor() {
    console.log(new.target.name);
  }
}

class Car extends Vehicle {
  constructor() {
    super();
  }
}

new Vehicle(); // Vehicle
new Car(); // Car

Consider the following use case: If we want the Vehicle class to be abstract, we can throw an error if (new.target.name === 'Vehicle') is true. However, you've to keep in mind that if you use this in production and build your project with bundlers, the names of your classes may be prefixed, causing the condition to always be false.

Comparison with Constructor functions

Before there were classes, constructor functions and prototypes were the default. I won't go too deep in this section, but i wanted to show you how we could achieve pretty much the same with constructor functions and prototypes since ES6 classes use prototypes behind the hood.

Properties and methods

Let's start by setting some properties and methods:

function Point(x, y) {
  this.x = x;
  this.y = y;

  this.translate = function(a, b) {
    this.x += a;
    this.y += b;
  }
}

const point = new Point(4, 5);
point.translate(2, 2);
point.x; // 6
point.y; // 7

Getters and Setters

To implement setters and getters we have to use Object.defineProperty or Object.defineProperties:

function Point(x, y) {
  this.x = x;
  this.y = y;

  Object.defineProperty(this, 'position', {
    set: function([x, y]) {
      [this.x, this.y] = [x, y];
    },
    get: function() {
      return [this.x, this.y];
    },
  });
}

const point = new Point();
point.position = [4, 5];
point.position; // [4, 5]

Basically, I used Object.defineProperty to set/change the property descriptor of the position property.

To learn more about property descriptors, you can check the next post.

Prototypal inheritance

Here's an example of prototypal inheritance:

function Vehicle(numberOfPassengers) {
  this.numberOfPassengers = numberOfPassengers;

  this.getNumberOfPassengers = function() {
    return this.numberOfPassengers;
  }
}

function Car() {
  Vehicle.call(this, 5); // The same way we used super for classes, here we call the Vehicle constructor in this context (Car context)
}

Car.prototype = Object.create(Vehicle.prototype); // Setting up the inheritance
Car.prototype.constructor = Car; // As a side effect of the line above, we loose the Car constructor. So we have to set it back

const car = new Car();
car.getNumberOfPassengers(); // 5

I won't go into much details here as there's a lot to talk about. But this is the minimal setup to do prototypal inheritance.

You may agree with me or not, but I find it a lot less straight forward and less descriptive than the class implementation.

Wrapping up

We covered a lot already. We saw all of the tools we can use to create classes that are tailored to our needs, we discussed how to create objects using classes and we talked about some caveats to be aware of.

Finally we saw how difficult it can be to use constructor functions compared to using classes.