How to Create Basic Inheritance in JavaScript Constructors
Creating inheritance between two JavaScript objects can be tricky business. Read on to learn how to solve this problem.
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Join For FreeThere are four ways to create an object in JavaScript. They are as follows:
- Object as literal
- Constructor Invocation Pattern
- The
create()
method - 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.
- It has duplicate code for
name
andage
initialization. It should use theanimal
constructor for this purpose. - 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.
- Using
__proto__
. - 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:
- Call the parent constructor using
call
orapply
. - 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.
Published at DZone with permission of Dhanajay Kumar, DZone MVB. See the original article here.
Opinions expressed by DZone contributors are their own.
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