Extending the Built-In JavaScript Type s in Visual Studio .NET

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Extending the Built-In JavaScript Type s
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Listing 22 Defining an Error Type
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ErrornonPositive = function(value) { var displayMessage = "NonPositive Exception: " + value; var e = Errorcreate(displayMessage, {"name": "NonPositive" } ); epopStackFrame(); return e; }
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Our nonPositive error is just a new object type It is considered a new Error type because it returns an Error object as its return value In the method body, the Errorcreate statement creates the Error object It creates it with an error message string, and then adds an object that contains additional information about the error Then, the epopStackFrame call attempts to attach the current call stack to the error
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popStackFrame
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The popStackFrame error has an effect only when the browser supports a stack property on the Error object Current browsers are Mozilla Firefox and Opera
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Finally, the error is returned as the function s return value
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We attached the nonPositive error to the global Error object, but doing this was just a design decision for this particular error We could have left it as a standalone object, or we could have attached it to some other namespace
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Now that we ve de ned our error, let s use it Listing 23 creates and throws the error if it determines that the value passed in is less than zero
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2: Microsoft AJA X Librar y Programming
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Listing 23 Throwing an Error
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function runTest(value) { try { if (value < 0) { throw ErrornonPositive(value); } } catch (e) { if (ename === "NonPositive") { alert (emessage); } } } runTest(-2); runTest(2);
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Extending the Microsoft AJAX Library
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The Microsoft AJAX Library was designed to be extensible so that it can be adapted to your application s needs We extend it by creating custom objects through a development pattern called the Prototype Model and the Library s type system We can extend it with new types of classes, interfaces, and enumerations The Prototype Model consists of four steps:
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1 Namespace declaration 2 Type declaration 3 Public interface declaration 4 Type registration
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Each individual step is straightforward, with the bulk of the programming work residing in the type and public interface declaration Let s look at each step in detail as we build a simple class
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Classes Classes are used to de ne objects that contain attributes and behaviors These attributes and behaviors are normally linked to form a logical grouping of information and behavior
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Extending the Microsoft AJA X Librar y Namespace Declaration
A namespace declaration is completely optional, just as it is in NET programming, but using it provides the same bene ts as it does in NET It helps group our types into logical blocks that are easier for us to comprehend and manage We might have a utilities namespace or a controls namespace Using a namespace also helps prevent naming collisions Creating a namespace is done through a single line of code The syntax for creating a namespace is as follows:
TyperegisterNamespace("namespace");
NOTE
The Type System
The type system (or Type object depending on how you look at it) is a new global object supplied by the Microsoft AJAX Library It s responsible for registering classes and namespaces, implementing inheritance and interfaces, and a whole host of other tasks It is similar in functionality to the type system in the NET Framework We use the type system extensively as we walk through creating new types
Listing 24 demonstrates a few different namespace declarations
Listing 24 Declaring Namespaces
TyperegisterNamespace ("BooksPublishers"); TyperegisterNamespace ("BooksAuthors"); TyperegisterNamespace ("SomeOtherNamespace");
There are two important points to understand when declaring namespaces First, declaring a namespace as we did with Typeregister Namespace("BooksPublishers") actually declares two namespaces: Books and BooksPublishers The outer namespace, Books, is implicitly created when we declare the more speci c namespace BooksPublishers This means that we don t have to explicitly declare the Books namespace to use it Figure 21 shows how Visual Studio s IntelliSense picks up the inner namespace s availability, although we didn t explicitly declare it, and Figure 22 shows the availability of the full namespace, BooksPublishers, that we explicitly declared
2: Microsoft AJA X Librar y Programming
Figure 21 IntelliSense in Visual Studio 2008 of an implicit namespace
Figure 22 IntelliSense in Visual Studio 2008 of a declared namespace
The second point is that registering the same namespace more than once is okay When you are working with a system of JavaScript les, more than one le might require a speci c namespace such as WebBehaviors Because the le creator won t know whether both les will always be used together, each le needs to declare the namespace within itself to guarantee its availability Because of this requirement, the Microsoft AJAX Library keeps track of which namespaces are already created and ignores namespaces that are declared more than once