Learn about inheritance in JavaScript (prototypal inheritance), behavior/object delegation pattern and objects linked to other objects.

NC Patro

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Feb 14, 2018

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What is Inheritance

Inheritance in most class-based object-oriented languages is a mechanism in which one object acquires all the properties and behaviors of another object. JavaScript is not a class-based language although class keyword is introduced in ES2015, it is just syntactical layer. JavaScript still works on prototype chain.

Classical Inheritance vs Prototypal Inheritance

Classical Inheritance (non-javascript)

• Vehicle is parent class and v1,v2 are the instances of Vehicle
• Car is child class of Vehicle and c1, c2 are instances of Car
• In classical inheritance it creates a copy of the behavior from parent class into the child when we extend the class and after that parent and child class are separate entity.
• Similarly, another copy of the behavior happens when we create an instance or object out of the class and both are separate entity again.
• It’s like car is manufactured from the vehicle and car blueprints but after that both are separate entity because they are not linked because It’s a copy. That’s the reason all arrows going downwards (property and behavior flowing down)

Prototypal Inheritance (Behavior delegation pattern)

• v1 and v2 are linked to Vehicle.prototype because it’s been created using new keyword.
• Similarly, c1 and c2 is linked to Car.prototype and Car.prototype is linked to Vehicle.prototype.
• In JavaScript when we create the object it does not copy the properties or behavior, it creates a link. Similar kind of linkage gets created in case of extend of class as well.
• All arrows go in the opposite direction compare to classical non-js inheritance because it’s a behavior delegation link. These links are known as prototype chain.
• This pattern is called Behavior Delegation Pattern which commonly known as prototypal inheritance in JavaScript.

You may go through article JavaScript — Prototype to understand the prototype chain in depth.

Example of prototypal inheritance

• Usage of Object.create() to achieve classical inheritance.
• In the below code snippet, Car.prototype and Vehicle.prototype is linked with help of Object.create() function.

// Vehicle - superclass
function Vehicle(name) {
  this.name = name;
}// superclass method
Vehicle.prototype.start = function() {
  return "engine of "+this.name + " starting...";
};// Car - subclass
function Car(name) {
  Vehicle.call(this,name); // call super constructor.
}// subclass extends superclass
Car.prototype = Object.create(Vehicle.prototype);// subclass method
Car.prototype.run = function() {
  console.log("Hello "+ this.start());
};// instances of subclass
var c1 = new Car("Fiesta");
var c2 = new Car("Baleno");// accessing the subclass method which internally access superclass method
c1.run();   // "Hello engine of Fiesta starting..."
c2.run();   // "Hello engine of Baleno starting..."

• In the above code, object c1 gets access to method run() and method start() because of below prototype chain. As per below diagram, we can see object c1 does not have these methods but it has links to go upwards.
• keyword this in above code is nothing but current execution context of each method which is c1 and c2.

To understand keyword this in details, you can go through article JavaScript — All about this and new keyword.

Diagrammatic representation of above code

Objects linked to other objects

• Now we will simplify the previous example code of inheritance with focusing only on objects and object linkages.
• So we will try to remove .prototype, constructor and new keyword, will be thinking about only objects.
• We will be using Object.create()function to create all the linkages between objects.

Below is simplified code of previous example.

// base object with methods including initialization
var Vehicle = {
  init: function(name) {
    this.name = name;
  },
  start: function() {
    return "engine of "+this.name + " starting...";
  }
}// delegation link created between sub object and base object
var Car = Object.create(Vehicle);// sub object method
Car.run = function() {
  console.log("Hello "+ this.start());
};// instance object with delegation link point to sub object
var c1 = Object.create(Car);
c1.init('Fiesta');var c2 = Object.create(Car);
c2.init('Baleno');c1.run();   // "Hello engine of Fiesta starting..."
c2.run();   // "Hello engine of Baleno starting..."

Diagrammatic representation of above code

• We can see now, how we have removed the complexity of new, all the .prototype, constructor functions and call method everything and still achieved the same result.
• Only thing which matters is object c1 linked to another object and then linked to another object again and so on.
• This is called object delegation pattern as well.

Summary

It is important to understand the prototypal inheritance and prototype chain before using these in code to avoid complexity.

Reference: You Don’t Know JS Book Series.

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