1: Programming with JavaScript Intervals in .NET framework

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1: Programming with JavaScript Intervals
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Intervals are identical to timeouts except that the function or string expression is continuously evaluated at the speci ed interval until the interval is canceled, instead of executing only one time when the delay expires We create an interval in Listing 143 using a windowsetInterval method that continually appends "hello!" to the div tag "abc"
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Listing 143 Using an Interval
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<body> <form id="form1" runat="server"> <div id="abc" /> </form> <script type="text/javascript"> var div = documentgetElementById("abc"); var intervalId = windowsetInterval( "divinnerHTML += 'hello!';", 1000); </script> </body>
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Figure 15 displays the effects of the interval executing seven times
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hello!hello!hello!hello!hello!hello!hello!
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Figure 15 Output of the abc div tag after 7 seconds
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Just as with timeouts, intervals can be canceled using the ID that is returned and the clearInterval method
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Listing 144 Clearing an Interval
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var intervalId = windowsetInterval("alert('x');", 1000); windowclearInterval(timeoutId);
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Other than that, there really aren t any other differences between an interval and a timeout The same rules apply to when an interval will execute and the intricacies of using expressions and functions
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Obje ct-Oriented JavaScript Programming
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Object-Oriented JavaScript Programming
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Because most ASPNET programmers have backgrounds in classic, classbased object-oriented systems, the Microsoft AJAX Library was designed to mimic a classic object-oriented system in JavaScript to provide a development experience most of its audience would nd welcoming The Microsoft AJAX Library implements new programming constructs to achieve this classic object-oriented system, but those constructs all rely on JavaScript language features, some of which were described in the preceding section, for their implementations These constructs are available for use without understanding how they were implemented, but it s our opinion that one can t successfully develop within the Microsoft AJAX Library, even with the new, more familiar constructs, without understanding the basics of how a classic object-oriented system can be created using out-of-the-box JavaScript So, in this section, we walk through how we can achieve two common object-oriented principles, abstract data types and inheritance in JavaScript, without using the Microsoft AJAX Library This should set you up for success when you start using the constructs provided by the Microsoft AJAX Library to create new systems
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A Word about JavaScript and Classic Object-Oriented Systems
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JavaScript is a prototypal object language It can be coerced into mimicking a classic system, but we re not advocating that mimicking a classic object-oriented system is in fact the best way to program with JavaScript In fact, some JavaScript purists believe that coercing JavaScript in this way is sacrilegious However, because this book is looking at JavaScript through the ASPNET AJAX lens and the Microsoft AJAX Library mimics a classic object-oriented system, it seems appropriate that we consider JavaScript in a classic system manner only rather than introduce other ways of working with it Furthermore, there are a few different approaches to coerce JavaScript so that it mimics a classic object-oriented system The way we describe in this section is just one way, and a not full-baked way at that A cursory Internet search on object-oriented JavaScript will show that opinions differ greatly about how best to achieve a classic system in JavaScript and that the idea is continually evolving
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1: Programming with JavaScript
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Abstract Data Types As stated earlier in the section Generally JavaScript, JavaScript doesn t support the class keyword as conventional object-oriented languages such as Java, C#, and C++ do Instead, JavaScript relies on functions having double duty as both procedural entry points and abstract data type de nitions In fact, every JavaScript programmer who has created a function has already created a new abstract data type Let s take a look at the simple example in Listing 145 The rst thing we do is declare a new empty function: Book Then we execute the function and assign it to the variable execResult Then we do something a bit different and use the Book function as an object constructor
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Listing 145 Defining Types
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function Book() {} var execResult = Book(); var myBook = new Book();
We can do this because the function keyword doubles as both a constructor function and a procedural method declaration This allows us to both create a new object using the function and execute it as a procedural method The new keyword determines which path the function keyword takes: Execute the method as a constructor or a procedural method that returns a result After we have an object, myBook, we can dynamically add properties to the instance as shown in Listing 146