Validating function parameters in VS .NET

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13.1.3 Validating function parameters
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JavaScript doesn t perform any kind of validation on the parameters passed to a method. Despite what happens in strongly typed languages like C# or VB.NET, where checks are performed at compile time, there s no guarantee that the parameters you receive in a method are of the expected type. There could also be fewer or more than the expected number.
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Figure 13.4 XML Comments added to custom JavaScript objects are used by the IntelliSense tool in Visual Studio Orcas.
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Implementing common Ajax patterns
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For example, consider a JavaScript function called add, which is supposed to return the sum of the two arguments it accepts:
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function add(a, b) { return a + b; }
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If the parameters a and b are numbers, everything goes as expected. But what happens if you pass strings instead of numbers In this case, the + operator is interpreted as the string-concatenation operator, and the function returns the string resulting from the concatenation of the strings passed as arguments. The function s code doesn t raise any errors; it just returns an unexpected result. Finally, if you pass arbitrary arguments to the function, such as a Boolean and a string, a JavaScript error is raised at runtime. Even if you can t prevent runtime errors from being raised, you can detect errors in the method and avoid returning unexpected results. An interesting feature introduced by the Microsoft Ajax Library is the ability to validate the parameters passed to a method. You can perform parameter validation through a method called Function._validateParams.
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Because Function._validateParams is declared as a private method, we feel the need to add a disclaimer that says use at your own risk. Used properly, this method can be a great help in some scenarios. We hope that the Function._validateParams method will become a publicly accessible method in the next release of the Microsoft Ajax Library.
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To illustrate how the method works, let s rewrite the add function to ensure that you operate on the right parameter types (see listing 13.4).
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Listing 13.4 Validating the parameters passed to a JavaScript function Listing 13.5
function add(a, b) { var e = Function._validateParams(arguments, [ {name:"a", type:Number, mayBeNull:false, optional:false}, {name:"b", type:Number, mayBeNull:false, optional:false} ] ); if(e) throw e; return a + b; }
Script versioning
The Function._validateParams method is called just after the function declaration, and its result is stored in the e variable. The first argument passed to the method is a variable called arguments; This is a special variable defined by JavaScript and accessible only in a function. The arguments variable holds a list of all the arguments passed to the function. The first argument passed to Function._validateParams is the list of method parameters to validate; these are the parameters passed to the add function when it s called. The second argument passed to Function._validateParams is an array containing two objects that act as parameter descriptors. Note that parameter descriptors are associated with method parameters in the same order they re stored in the arguments variable: The first descriptor is associated with the first parameter passed to the function, and so on. Each descriptor describes a particular parameter by specifying a list of requirements that it must meet. These requirements are represented by properties of the object. For example, look at the first parameter descriptor supplied:
{name:"a", type:Number, mayBeNull:false, optional:false}
The name property contains a string with the parameter name. If the validation fails, you can access the name of the parameter through the Error object. The second property, type, puts a constraint on the type of the parameter. In this case, you mandate that the a parameter is of type Number. The third property, mayBeNull, determines whether you can pass null as the value of the argument. If the property is set to false, you can t pass null as the value for the a parameter. The last property, optional, specifies whether the parameter is mandatory. If you set the property to false, the caller must supply at least one parameter, because you re dealing with the first parameter descriptor. The properties illustrated in the previous example aren t the only ones available. Table 13.1 lists the properties you can declare in a parameter descriptor.
Table 13.1 Properties used in parameter descriptors Description The expected type of a parameter. The possible values are String, Number, Array, Function, and Object. If true, the parameter can be null. If true, the parameter can be omitted. If the parameter is a Number, specify if it must be an integer.
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