How to Create Basic Inheritance in JavaScript Constructors

There are four ways to create an object in JavaScript. They are as follows:

1. Object as literal
2. Constructor Invocation Pattern
3. The create() method
4. Using class after ES6

The implementation of inheritance varies according to the object creation method. In this post, I am going to explain creating inheritance between a function constructor.

Let’s say you have a function:

function animal(name, age) {
    this.name = name;
    this.age = age;
}

If you call the animal function using the new operator, an object will be created. This way creating objects is also known as the “Constructor Invocation Pattern.”

var dog = new animal('foo', 5);
console.log(dog);
var cat = new animal('koo', 3);
console.log(cat);

The dog and cat objects both have their own names and age properties. If you want a property or method to be shared across all objects, add that to the prototype of the function.

animal.prototype.canRun = function () {
    console.log('yes ' + this.name + ' can run !');
}

Using the JavaScript prototype chain, both the dog and cat objects can access the canRun method.

var dog = new animal('foo', 5);
dog.canRun(); // yes foo can run

var cat = new animal('koo', 3);
cat.canRun(); // yes koo can run

Next, let us create another constructor – human:

function human(name, age, money) {
    this.name = name ;
    this.age = age ; 
    this.money = money;
}
human.prototype.canEarn = function () {
    console.log('yes ' + this.name + 'can earn');
}

At this point in time, the human and animal functions do not have any relationship. However, we know that humans are also animals. There are two problems with the human constructor.

1. It has duplicate code for name and ageinitialization. It should use theanimal constructor for this purpose.
2. It does not have any links with the animal constructor.

The above problems can be removed by creating inheritance between the animal and human function constructors.

You can solve Problem 1 of the code duplication issue by modifying the human function as shown below:

function human(name, age, money) {
    animal.call(this, name, age);
    this.money = money;
}

Now, in the human function, we are using the call method to manually pass a current object as a value of this in the animal function. This approach is also called the Indirect Invocation Pattern. Now, an object instance for human can be created as shown below:

var h1 = new human('dj', 30, '2000 $');
console.log(h1); 

So far, we have solved Problem 1 of our code duplication issue; however, the human function is still not linked to the animal function. If you try to call the canRun method on the  h1 object, JavaScript will throw you an error.

h1.canRun(); // throw error canRun is not a function 

You can fix this problem by linking the prototype of the human function with the prototype of theanimal function constructor. There are two ways to do that.

1. Using __proto__. 
2. Using the Object.create() method.

You can link the prototype of the function constructors using Object.create() as shown below:

human.prototype = Object.create(animal.prototype);

You can link the prototype of the function constructors using __proto__ as shown below:

human.prototype.__proto__ = animal.prototype;

I would prefer the Object.create() method because __proto__ may not be supported in many browsers. After linking the prototypes in one way, you have created inheritance between the animal and human function constructors. The object instance of human can read all the properties of the animal function and execute theanimal function's methods.

For your reference, the full source code to implement inheritance between function constructors is listed below:

function animal(name, age) {
    this.name = name;
    this.age = age;
}

animal.prototype.canRun = function () {
    console.log('yes ' + this.name + ' can run !');
}

var dog = new animal('foo', 5);
dog.canRun();

var cat = new animal('koo', 3);
cat.canRun();
function human(name, age, money) {
    animal.call(this, name, age);
    this.money = money;
}

human.prototype = Object.create(animal.prototype);

human.prototype.canEarn = function () {
    console.log('yes ' + this.name + 'can earn');
}
// human.prototype.__proto__ = animal.prototype;
var h1 = new human('dj', 30, '2000 $');
h1.canRun();
h1.canEarn();

To create inheritance between function constructors, always perform the following two actions:

1. Call the parent constructor using call or apply.
2. Link the prototype of the child constructor to the parent constructor prototype.

I hope now you understand how to implement inheritance between function constructors in JavaScript.