Object-oriented JavaScript
by Greg Brown | Pages: 1, 2, 3

Polymorphism

Polymorphism refers to the ability of a caller to invoke a particular method or set of methods on an object without regard to the object's type. For example, we might want to add a speak() method to our Pet class, to allow our Pets to answer the phone when we are not at home; the caller on the other end of the line will not necessarily know or care which Pet is answering the phone, as long as it is able to speak():

In C#:

// C# Pet class

public class Pet
{
    // ...

    public virtual void Speak()
    {
        System.Console.WriteLine(GetName() + " says...");
    }
}

// C# Dog class

public class Dog : Pet
{
    // ...

    public override void Speak()
    {
        base.Speak();
        System.Console.WriteLine("woof");
    }
}

// C# Cat class

public class Cat : Pet
{
    // ...

    public override void Speak()
    {
        base.Speak();
        System.Console.WriteLine("meow");
    }
}

In JavaScript:

// JavaScript Pet class
// ...

Pet.prototype.speak = function() {
    alert(this.getName() + " says...");
}


// JavaScript Dog class
// ...

Dog.prototype.speak = function() {
    Pet.prototype.speak.call(this);
    alert("woof");
}


// JavaScript Cat class
// ...

Cat.prototype.speak = function() {
    Pet.prototype.speak.call(this);
    alert("meow");
}

Note that the same call() function used to invoke the base class constructor functions is also used to invoke method on the base class:

Pet.prototype.speak.call(this);

In this case, however, the target function name is the fully qualified name of the base class method, Pet.prototype.speak.

Invoking the methods is the same as before:

C#:

p = new Dog("Max");
p.Speak();

p = new Cat("Fluffy");
p.Speak();

JavaScript:

p = new Dog("Max");
p.speak();

p = new Cat("Fluffy");
p.speak();

The output of this program should be the following (see Figures 3-6):

Figure 3. Output is "Max says..."

Figure 4. Output is "woof"

Figure 5. Output is "Fluffy says..."

Figure 6. Output is "meow"

Although it may not offer features as powerful as C# or Java, JavaScript is more capable than many web developers may know, and it can be used to provide the structure of object-oriented development to the growing number of AJAX applications currently being deployed on the web.

---

---

2. Great help
   2009-11-08 21:59:50 Abhijit Roychoudhury
4. Good and basic article
   2008-05-30 10:46:26 Atul Jindal
6. Douglas Crockford would disagree
   2006-06-08 06:27:20 joshua.gitlin@gmail.com
8. Nice articles, but a major imprecision
   2006-06-08 00:17:16 andrea.campi
• Nice articles, but a major imprecision
  2006-06-08 06:47:49 Greg.Brown
• Nice articles, but a major imprecision
  2006-06-08 07:23:35 jdodds
• Nice articles, but a major imprecision
  2006-06-08 07:40:35 Greg.Brown