Object-Oriented JavaScript

In this chapter, you'll learn about OOP (Object-Oriented Programming) and how it relates to JavaScript. As an ASP.NET developer, you probably have some experience working with objects, and you may even be familiar with concepts such as inheritance.

However, unless you're already an experienced JavaScript programmer, you probably aren't familiar with the way JavaScript objects and functions really work. This knowledge is necessary in order to understand how the Microsoft AJAX Library works, and this chapter will teach you the necessary foundations. More specifically, you will learn:

• What encapsulation, inheritance, and polymorphism mean
• How JavaScript functions work
• How to use anonymous functions and closures
• How to read a class diagram, and implement it using JavaScript code
• How to work with JavaScript prototypes
• How the execution context and scope affect the output of JavaScript functions
• How to implement inheritance using closures and prototypes
• What JSON is, and what a JSON structure looks like

In the next chapters you'll use this theory to work effectively with the Microsoft AJAX Library.

Concepts of Object-Oriented Programming

Most ASP.NET developers are familiar with the fundamental OOP principles because this knowledge is important when developing for the .NET development. Similarly, to develop client-side code using the Microsoft AJAX Library, you need to be familiar with JavaScript's OOP features. Although not particularly difficult, understanding these features can be a bit challenging at first, because JavaScript's OOP model is different than that of languages such as C#, VB.NET, C++, or Java.

JavaScript is an object-based language. Just as in C#, you can create objects, call their methods, pass them as parameters, and so on. You could see this clearly when working with the DOM, where you manipulated the HTML document through the methods and properties of the implicit document object. However, JavaScript isn't generally considered a fully object-oriented language because it lacks support for some features that you'd find in "real" OOP languages, or simply implements them differently.

Your most important goal for this chapter is to understand how to work with JavaScript objects. As an ASP.NET developer, we assume that you already know how OOP works with .NET languages, although advanced knowledge isn't necessary. A tutorial written by Cristian Darie on OOP development with C# can be downloaded in PDF format at http://www.cristiandarie.ro/downloads/.

To ensure we start off from the same square, in the following couple of pages we'll review the essential OOP concepts as they apply in C# and other languages—objects, classes, encapsulation, inheritance, and polymorphism. Then we'll continue by "porting" this knowledge into the JavaScript realm.