Showing posts with label inheritance. Show all posts
Showing posts with label inheritance. Show all posts

Tuesday, April 07, 2015

Simple Inheritance with JavaScript

Simple Inheritance with JavaScript

JavaScript uses a different approach than C# or C++ to create an object-oriented language. It is a prototype-based language. The concept of prototyping implies that behavior can be reused by cloning existing objects that serve as prototypes. Every object in JavaScript decends from a prototype which defines a set of functions and members that the object can use. There is no class. Just objects. Every object can then be used as a prototype for another object.

This concept is extremely flexible and we can use it to simulate some concepts from OOP like inheritance.

Implementing Inheritance

Let's visualize what we want to create with this hierarchy using JavaScript:

Simple Inheritance with JavaScript

First of all, we can create ClassA easily. Because there are no explicit classes, we can define a set of behavior (A class so…) by just creating a function like this:

var ClassA = function() {
    this.name = "class A";
}

This "class" can be instantiated using the new keyword:

var a = new ClassA();
ClassA.prototype.print = function() {
    console.log(this.name);
}

And to use it using our object:

a.print();

Fairly simple, right?

The complete sample is just 8 lines long:

var ClassA = function() {
    this.name = "class A";
}
 
ClassA.prototype.print = function() {
    console.log(this.name);
}
 
var a = new ClassA();
 
a.print();

Now let's add a tool to create "inheritance" between classes. This tool will just have to do one single thing: clone the prototype:

var inheritsFrom = function (child, parent) {
    child.prototype = Object.create(parent.prototype);
};

This is exactly where the magic happens! By cloning the prototype, we transfer all members and functions to the new class.

So if we want to add a second class that will be a child of the first one, we just have to use this code:

var ClassB = function() {
    this.name = "class B";
    this.surname = "I'm the child";
}
 
inheritsFrom(ClassB, ClassA);

Then because ClassB inherited the print function from ClassA, the following code is working:

var b = new ClassB();
b.print();

And produces the following output:

class B

We can even override the print function for ClassB:

ClassB.prototype.print = function() {
    ClassA.prototype.print.call(this);
    console.log(this.surname);
}

In this case, the produced output will look like this:

class B 
I'm the child

The trick here is to the call ClassA.prototype to get the base print function. Then thanks to call function we can call the base function on the current object (this).

Creating ClassC is now obvious:

var ClassC = function () {
    this.name = "class C";
    this.surname = "I'm the grandchild";
}
 
inheritsFrom(ClassC, ClassB);
 
ClassC.prototype.foo = function() {
    // Do some funky stuff here...
}
 
ClassC.prototype.print = function () {
    ClassB.prototype.print.call(this);
    console.log("Sounds like this is working!");
}
 
var c = new ClassC();
c.print();

And the output is:

class C 
I'm the grandchild 
Sounds like this is working!

More Hands-on with JavaScript

It might surprise you a bit, but Microsoft has a bunch of free learning on many open source JavaScript topics and we’re on a mission to create a lot more with Project Spartan coming. Check out my own:


And some free tools: Visual Studio Community, Azure Trial, and cross-browser testing tools for Mac, Linux, or Windows.