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IntelliSense in Visual Studio 2008
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IntelliSense in Visual Studio 2008 has improved dramatically We cover it in detail in Appendix A, JavaScript in Visual Studio 2008
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Type Declaration
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In pure technical terms, a type declaration is a variable name assigned to a Function object whose purpose is to act as a constructor method and create an object of that type (a concept we covered in 1) The type declaration also contains members that should be created and assigned to each instance of that type and are meant to be accessed through the public
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interface Listing 25 de nes a new type, Publisher, using the Books Publishers namespace we declared in Listing 24 The code also attaches two new members to the type: _name and _city
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Listing 25 Defining a Type
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BooksPublishersPublisher = function() { this_name = null; this_city = null; }
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Classes Are Functions!
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The importance of understanding that a class is a variable assigned to a Function object is something that we can t stress enough It is perhaps the most crucial piece of information you need to understand to comprehend the Prototype Model
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NOTE
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Capitalizing Constructor
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When de ning methods that are intended to be used as constructor methods, we use PascalCase, resulting in each word in the function s name being capitalized
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We can also pass in parameters to the constructor method as shown in Listing 26
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Listing 26 Defining a Constructor with Parameters
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BooksPublishersPublisher = function(name, city) { this_name = name; this_city = city; }
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Privacy
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When de ning types in this manner, there is no real concept of privacy Our want-to-be private members can be accessed from an instance of that type just by requesting the member name We can t declare the name and city members we just de ned as private, as we can in NET, and have the JavaScript runtime enforce the privacy The best we can do using this
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2: Microsoft AJA X Librar y Programming
style of programming is have privacy by convention Privacy by convention is naming things in a certain way such that when a tool such as IntelliSense or a person reads the code, it is understood that the variable was intended to be private In the case of ASPNET AJAX, members and functions that are pre xed with an underscore (_) are considered private members, and their use by other objects should be avoided If you do decide to access a private member or method, understand that its implementation may change in future releases or be completely removed We should use private members and methods with the same consideration and care that we access private members and methods in NET through re ection
Finally, in the case that our type inherits from another type, we need to call the base class s constructor when our constructor is executed This is done using the initializeBase method, as shown in Listing 27
Listing 27 Initializing Inheritance
BooksPublishersPublisher = function(name, city) { BooksPublishersPublisherinitializeBase(this, [name]); this_name = name; this_city = city; }
The initializeBase method takes as a parameter the pointer to the current object, this, and an argument array In cases where there are no arguments to pass to the base type s constructor, the argument array can be set to null or not passed in at all In other cases, we can selectively pass the arguments our base class requires, as we do in the Listing 27, by creating a new array
Executing initializeBase
Even if our type doesn t inherit from another type, executing initializeBase is recommended It won t hurt anything, and because we de ne what our type inherits from in another section of code, it s possible that we ll change its inheritance at a later point and forget to include the initializeBase method If we just include it from the beginning, we won t have this problem
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Extension of the Function Object
The initializeBase method is available to our BooksPublishers Publisher class because in the Microsoft AJAX Library, the Function object type s prototype has been extended to include the initializeBase method Because our BooksPublishersPublisher object is a pointer to a Function object, the initializeBase method is available to it (The Microsoft AJAX Library does this in a roundabout way using the previously mentioned type system to extend the Function s prototype, but it has the same effect as directly extending the Function s prototype)