1: Programming with JavaScript in .NET

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1: Programming with JavaScript
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One advantage of using a function, either anonymous inline or a prede ned, instead of a string expression is avoiding the complexity of concatenating a string In Listing 138, we create two timeouts that produce the same result The rst uses the string concatenation method, and the second uses an anonymous function
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Listing 138 Explicit Anonymous Function
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var a = new Date(); var b = "Hello!"; windowsetTimeout ("alert ('" + b + " today is " + a + "');", 1000); windowsetTimeout( function () { alert (b + " today is " + a); }, 1000);
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As Listing 138 shows, because we re trying to use a string in the code we want to register, the string concatenation needed to create the valid string expression gets complicated very quickly even when working with our simple example On the other hand, using a function allows us to program normally and keep the code readable When we execute the code in Listing 138, we receive two identical alerts that look something like what s shown in Figure 13
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Figure 13 Output of alert registered with timeout
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There are some dif culties, however, with using a function rather than a string The dif culties lie in what the parameter values are when the method executes after the delay
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The fact that the two alerts created by Listing 138 are identical is actually just luck The only reason they re identical is because the variables do not change once we create the second timeout We can see how fragile the identical methods are by altering our code slightly Listing 139 adds a statement that updates variable b to "boo!" once we create the second timeout
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Listing 139 Problems with Functions and Variables
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var a = new Date(); var b = "Hello!"; windowsetTimeout ("alert ('" + b + " today is " + a + "');", 1000); windowsetTimeout( function () { alert (b + " today is " + a); }, 1000); b = "Boo!";
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Now when our timeouts execute they do not produce the same alerts The rst timeout still produces the same alert shown in Figure 13, but the second timeout produces an alert similar to Figure 14
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Figure 14 Output of alert after we modified variable b
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The reason this happens is because of a subtlety in how the string expression treats variables As mentioned earlier, when JavaScript encounters the string, it wraps the string in an anonymous method, and that s what is executed when the delay expires When JavaScript creates this anonymous method and there are variables included in the string used to create the method as there are in our example, it evaluates those parameters at the time that the function is created So, rather than create a function that
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1: Programming with JavaScript
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looks similar to the one used in the second setTimeout method, it looks like the one shown in Listing 140
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Listing 140 Automatically Generated Anonymous Function
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function anonymous() { alert ('Hello! today is Sun Feb 17 21:28:36 PST 2008'); }
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Now when our timeout expires it alerts the predetermined string, and even though we ve modi ed the b s value before the timeout expires, the modi cation has no effect In comparison, when a function is used, the function attached to the timeout and the variable references inside the function are evaluated when the delay expires and the function executes This different execution pattern means that the variables used inside the function could have been modi ed since the timeout was created As you might have guessed, this could be an undesirable characteristic of our timeout because we might want our timeout to use the variable s value as it was when the timeout was created Yet, we also don t want to resort to using a string for our timeout, because it s hard to work with Don t worry; there is a way around this problem First, let s x the anonymous function version of our code, and then we can cover how to x the prede ned function version If we wrap our setTimeout in a function and then immediately execute it, we can create scope and thus remember the original values of our variables Listing 141 demonstrates how we can x the anonymous function method
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