Defining Prototype-Based Methods in VS .NET

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Listing 153 Defining Prototype-Based Methods
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Book = function (publisher, subject, publishYear) { thispublisher = publisher; thissubject = subject; var publishYear = publishYear; WasPublishedThisYear = function() { var currdate = new Date(); return (currdategetFullYear() === publishYear); } }
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1: Programming with JavaScript
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Listing 153 continued
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BookprototypeIsSubjectDotNet = function() { return (thissubjectindexOf('NET') !== -1); } BookprototypeIsBookNewAndCool = function() { return (WasPublishedThisYear() && thisIsSubjectDotNet ()); } var myFinalBook = new Book('A&W', 'NET', 2007); // alerts "true" alert (myFinalBookIsSubjectDotNet()); // alerts "false" alert (myFinalBookIsBookNewAndCool());
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Now, whenever a new Book instance is created, its methods are cloned from its prototype Using the prototype method is the best performing way to create methods and assign them to an abstract data type s de nition because only one copy of the methods is created and stored versus creating them for each object instance
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Prototyping in a Prototype Language
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The concept of prototyping is the major tenet of prototype-based languages, of which JavaScript is one In prototype-based languages, objects are not instantiated by creating an instance of a particular class from a class de nition, but rather are cloned from existing objects and thus copy the behavior (the prototype) of the existing object, preserving the same qualities as the original To create a new object type, we modify the prototype of an object and then clone it as needed In JavaScript, we modify an object s prototype by manipulating a special property called prototype This is what we did in the example in Listing 153 An interesting property of prototypes is that when we manipulate an object s prototype, all instances of that object recognize the change immediately For instance, if we add a new function to our Book s prototype, when we have an instance of the object the instance of the object will be able to access the new function Listing 154 demonstrates this idea as we add a new method, toArray, and access it from our previously created myFinalBook variable
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Listing 154 Modifying an Existing Object s Prototype
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// create the object var myFinalBook = new Book('A&W', 'NET', 2007); // modify the prototype BookprototypetoArray = function() { return [thispublisher, thissubject]; } // new method is available on object alert (myFinalBooktoArray()length);
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A Word about Closures
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The Abstract Data Types section we just went over relies heavily on the idea of closures We didn t mention them by name because explaining them and how they work can be a bit dif cult, and we really wanted to relate this section to an object-oriented principle with which you might be more familiar For those who care, however, a closure is de ned as a method that contains local variables and inner functions that still exist after the outer method has completed its execution Put into more practical terms, a closure is implemented by having one function contain another with a pointer to the inner function being available after the outer function has exited The IsBookNewAndCool function in Listing 152 is an example of an inner function declared inside an outer one that s available after the Book constructor exited, and thus a new closure was formed when we created an instance of the Book type Important to note is that a new closure is formed every time we create a Book instance, and hence the memory footprint of a creating a Book closure that contains a lot of inner functions is large as we explained in the discussion of Listing 152 Closures can be used for many different programming tasks other than what we ve used them for in this section and also have some serious caveats when it comes to garbage collection, performance, and usage For more information, we suggest reading the excellent blog entry at http://blogmorrisjohnscom/javascript_closures_for_dummies For the more theoretical ideas, check out wwwjibberingcom/faq/ faq_notes/closures html
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