Classes

Constructor functions

We often need to create many objects of the same kind, like users or cars. In object-oriented languages, a class is often used for this purpose. JavaScript, being a prototype-based language, has something called constructor functions.

// ES5 constructor function
function Car(brand) {
    this.brand = brand;
}

// Adding a method to the prototype
Car.prototype.getBrand = function() {
    return this.brand;
}

Classes, introduced with ES6, provide “syntactical sugar,” which is comparable to inheritance in object-oriented languages. Classes are special functions meant to mimic the class keyword from these other languages.

Defining a class

To define a class, we use the class keyword followed by the name of the class. The body of a class is the part that is enclosed in curly braces {}:

class Car {
  // Body of the class
}

The code above defines a class named Car. This syntax is named class declaration.

The new operator instantiates the class:

const myCar = new Car();

This is how we create a new object, an instance, of the Car class.

console.log(typeof Car); // -> function

As you can see, a class is a kind of function in JavaScript.

We don’t need to give the class a name. By using a class expression, you can assign the class to a variable:

const Car = class {} // unnamed
const Car = class Car {} // named

Class expressions can be named or unnamed. The name given to a named class expression is local to the class’s body.

Hoisting

An important difference between function declarations and class declarations is that the function declarations are hoisted, whereas the class declarations are not. Therefore, you need to first declare the class before you can initialize objects with the new keyword.

“Conceptually, for example, a strict definition of hoisting suggests that variable and function declarations are physically moved to the top of your code, but this is not what happens. Instead, the variable and function declarations are put into memory during the compile phase but stay exactly where you typed them in your code.” – MDN

As you might remember, variables declared with var are also hoisted. Keeping with the “modern” way, it makes then sense that classes, just like variables declared with let or const, are not hoisted.

Initialization

Classes in JavaScript have a special method called constructor that lets us set initial values for fields when the object is initialized. There can be only one constructor method.

Let’s create a Car class with a constructor that sets the initial value of the field brand:

class Car { 
  constructor(brand) {
    this.brand = brand;    
  } 
}

The expression this.brand = brand creates a public field brand and assigns it an initial value. We can now create a new car object and access its brand property:

const car = new Car('Volvo');

console.log(car.brand); // -> Volvo

As you can see, we can now access the brand field using a property accessor. There’s no restriction to access the public fields. You can read and modify the values of public fields inside the constructor and methods and outside of the class.

The bodies of class declarations and class expressions are executed in strict mode.

Methods

Classes provide a cleaner and more elegant syntax for adding methods.

class Car { 
  constructor(brand) {
    this.brand = brand;
  } 

  getBrand() {
    return this.brand;
  }
}

The this keyword allows us to access instance data inside the constructor and methods.

const car = new Car('Tesla');
console.log(car.getBrand()); // -> Tesla

The method invocation car.getBrand() executes the methods inside the Car class and returns the brand property.

Static methods

We can also assign a method to the class function itself, not to its prototype. Such methods are called static methods and they are prepended with the static keyword:

class Double {
  static double(n) {
    return n * 2;
  }
}

Double.double(2); // -> 4

We don’t have to create an instance of the class but can call the method directly on the class itself. Static methods are often used in utility classes for making computations.

Getters and setters

JavaScript classes can have getter and setter functions.

class Car { 
  constructor(brand) {
    this._brand = brand;
  } 

  get brand() {
    return this._brand;
  }

  set brand(value) {
    this._brand = value;
  }
}

Notice that if you run this.brand = value in the setter we would be calling the setter again and go into an infinite loop. Using an underscore _ is a common convention.

With this, we can create an instance of the class and access the brand property through the setter and getter:

const car = new Car('');
car.brand = 'Tesla';
console.log(car.brand); // -> Tesla

Inheritance

Class inheritance is a way to inherit the functionality of another class and extend it. We create a child class with the extends keyword.

class SportsCar extends Car {
  isFast() {
    return true;
  }
}

Now, we can use the functionality of the class we extended:

const car = new SportsCar('Mazda');
console.log(car.brand); // -> Mazda
console.log(car.isFast()); // -> true

You can see here that we didn’t have to create a constructor. If a class extends to another class and has no constructor, the following constructor is generated:

constructor(...args) {
  super(...args);
}

The super keyword is used to call corresponding methods of the parent class. So, the constructor calls the parent constructor passing all the arguments. If we want to add another in-parameter to our class, we can create a constructor:

class SportsCar extends Car {
  constructor(brand, model) {
    super(brand);
    this.model = model;
  }
}

const myCar = new SportsCar("Mazda", "MX-5");

Note that constructors in inheriting classes must call super() before using this.

Conclusion

• Use classes when you need to create multiple objects of the same kind.

• Classes are a special kind of function that runs in strict mode and is not hoisted.

• Classes can have fields and methods that we can access from a class instance.

• We can inherit functionality from other classes by extending them.

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Quiz on classes