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Inheritance use none none writer toprint none on none GS1 DataBar Inheritance i none for none s a very elegant way to reuse code that has already been written. For example, you can have a generic object Person, which has properties such as name and date of birth, and that implements the functionality walk, talk, sleep, eat. Then you figure out that you need an object Programmer.

You could re-implement all the methods and properties that Person has, but it would be smarter to just say that Programmer inherits Person, and save yourself some work. The Programmer object only needs to implement more-specific functionality, such as the method "write code", while reusing all of the Person"s functionality. In classical OOP, classes inherit from other classes, but in JavaScript, because there are no classes, objects inherit from other objects.

When an object inherits from another object, it usually adds new methods to the inherited ones, thus extending the old object. Often the following phrases can be used interchangeably: "B inherits from A" and "B extends A". Also, the object that inherited a number of methods, can pick one or more methods and redefine them, customizing them for its own needs.

This way the interface stays the same, the method name is the same, but when called on the new object, the method behaves differently. This way of redefining how an inherited method works is known as overriding..

[ 15 ]. Introduction Polymorphism In the exampl none none e above, we had a Programmer object that inherited all of the methods of the parent Person object. This means that both objects provide a "talk" method, among others. Now imagine that somewhere in our code, there"s a variable called Bob and it so happens that we don"t know if Bob is a Person, or a Programmer object.

We can still call the "talk" method on the Bob object and the code will work. This ability to call the same method on different objects and have each of them respond in their own way is called polymorphism..

OOP Summary If you are ne none none w to the OO programming lingo and you"re not sure you"ve fully grasped the concepts above, don"t worry. We"ll look at some code and you"ll see that, although they may seem complicated when just talking about high-level concepts, things are much simpler in practice. Thus said, let"s rehash the concepts once more.

. Feature Bob i s a man (an object). Bob"s date of birth is June 1st, 1980, gender: male, hair: black. Bob can eat, sleep, drink, dream, talk and calculate his age.

Bob is an instance of class Programmer. Bob is based on another object, called Programmer. Bob holds data (such as birth date) and methods that work with the data (such as calculate age).

Illustrates concept objects properties methods class (in classical OOP) prototype (in prototypal OOP) encapsulation. We don"t need none none to know how the calculation method works information hiding internally. The object might have some private data, such as the number of days in February in a leap year, we don"t know, nor do we want to know. Bob is part of a Web Dev Team object, together with Jill, a Designer object and Jack, a Project Manager object.

aggregation, composition. [ 16 ]. 1 . Feature Designer, Project Manager and Programmer are all based on and extend a Person object. Illustrates concept inheritance You can call the methods Bob:talk, Jill:talk and Jack:talk and polymorphism, method they"ll all work fine, albeit producing different results (Bob will overriding probably talk more about performance, Jill about beauty and Jack about deadlines). Each object inherited the method talk from Person and customized it..

Setting up Your Training Environment This book tak es a "do it yourself" approach when it comes to writing code, because the author firmly believes that the best way to really learn a programming language is by writing code. So there"s no cut-and-paste-ready code downloads, which you can simply put in your pages. On the contrary, you"re expected to type in code, see how it works and then tweak it and play around with it.

When trying out the code examples, you"re encouraged to enter the code into Firebug"s console. Let"s see how you go about doing this..

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