How to inherit methods in JavaScript?

Published June 25, 2020

We already discussed inheriting properties in a previous post on How to inherit properties in JavaScript?

Today we will be discussing on inheriting methods in JavaScript.

Consider our old class Vehicle from our previous post.

// Vehicle base class
const Vehicle = function (type, year, numberOfTyres) {
  this.type = type;
  this.year = year;
  this.numberOfTyres = numberOfTyres;
};

Let's now add a method called start() to the Vehicle class.

It is better and a convention to add methods to the prototype chain of the class.

// Vehicle base class
const Vehicle = function (type, year, numberOfTyres) {
  this.type = type;
  this.year = year;
  this.numberOfTyres = numberOfTyres;
};

// add a method to the prototype chain
Vehicle.prototype.start = () => {
  console.log("Vehicle started");
};

Ok. Everything is fine till now. 😃

Now, let's make another class called Car and inherit all the properties as well as our methods from the Vehicle base class.

// Vehicle base class
const Vehicle = function (type, year, numberOfTyres) {
  this.type = type;
  this.year = year;
  this.numberOfTyres = numberOfTyres;
};

// add a method to the prototype chain
Vehicle.prototype.start = () => {
  console.log("Vehicle started");
};

// Car class
const Car = function (name, type, year, numberOfTyres) {
  Vehicle.call(this, type, year, numberOfTyres);
  this.name = name;
};

If you recall from our previous post on How to inherit properties in JavaScript?, then you would know that the Vehicle.call() is the method used to inherit properties from the Vehicle base class.

Let's create an object called Audi.

// Vehicle base class
const Vehicle = function (type, year, numberOfTyres) {
  this.type = type;
  this.year = year;
  this.numberOfTyres = numberOfTyres;
};

// add a method to the prototype chain
Vehicle.prototype.start = () => {
  console.log("Vehicle started");
};

// Car class
const Car = function (name, type, year, numberOfTyres) {
  Vehicle.call(this, type, year, numberOfTyres);
  this.name = name;
};

// create object Audi
const Audi = new Car("Audi", "Car", 2020, 4);

But the problem here is that we cannot invoke the start() method which is defined in the Vehicle class. It is because the start() method is not inherited using the call() method from Vehicle class. Only properties are inherited.

So, let's inherit the methods.

To inherit methods first we need to point our Car class prototype to our Vehicle base class prototype.

Currently, the Car class prototype is pointing to itself.

We achieve this using the Object.create() static method.

Note: You should this before creating an object from our Car class.

// Vehicle base class
const Vehicle = function (type, year, numberOfTyres) {
  this.type = type;
  this.year = year;
  this.numberOfTyres = numberOfTyres;
};

// add a method to the prototype chain
Vehicle.prototype.start = () => {
  console.log("Vehicle started");
};

// Car class
const Car = function (name, type, year, numberOfTyres) {
  Vehicle.call(this, type, year, numberOfTyres);
  this.name = name;
};

// point prototype of Car class to Vehicle class
Car.prototype = Object.create(Vehicle.prototype);

// create object Audi
const Audi = new Car("Audi", "Car", 2020, 4);

Ok. One more thing. 🦄

When we changed our prototype of Car class to Vehicle class prototype, the constructor of Car was also changed in the process.

We need to change it back to the Car constructor otherwise it may cause errors in the long run.

Let's achieve this using the Object.defineProperty.

// Vehicle base class
const Vehicle = function (type, year, numberOfTyres) {
  this.type = type;
  this.year = year;
  this.numberOfTyres = numberOfTyres;
};

// add a method to the prototype chain
Vehicle.prototype.start = () => {
  console.log("Vehicle started");
};

// Car class
const Car = function (name, type, year, numberOfTyres) {
  Vehicle.call(this, type, year, numberOfTyres);
  this.name = name;
};

// point prototype of Car class to Vehicle class
Car.prototype = Object.create(Vehicle.prototype);

// change the constructor back to Car class constructor
Object.defineProperty(Car.prototype, "constructor", {
  value: Car,
  enumerable: false,
  writable: true,
});

// create object Audi
const Audi = new Car("Audi", "Car", 2020, 4);

Note:

  • The Object.defineProperty() creates a new value called constructor in the Car class prototype and put its value to the Car class itself, just what we needed.

  • Setting the enumerable property to false helps to avoid it being shown while looping.

  • Setting the writable property to true makes it mutable.

Now we can invoke the start() method from our Audi object.

Audi.start(); // Vehicle started

Feel free to share if you found this useful 😃.


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