itextsharp barcode vb net 7: Debugging DOM Applications in Java

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7: Debugging DOM Applications
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The basic rules to follow here are simple: first, don t put code that must always execute into ASSERT() calls; and second, use ASSERT() to catch bugs, not garden-variety errors that are going to happen in the normal course of the program
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Getting Fancy with ASSERT()
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Once you get used to the idea of how ASSERT() works and where to effectively use it, you can define a whole bunch of useful, special-purpose ASSERT() functions that perform very specific tasks For example, you can write an ASSERT() that checks to see if a given node is indeed a valid node that is, that the nodeType property actually exists (in the JavaScript case) and it is between 1 (for Element nodes) and 12 (for Notation nodes) The ASSERT() function for this in JavaScript looks like this:
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function ASSERT_VALID_NODE(objNode) { if (objNode == null) { alert("Invalid node -- node is null"); return; } if (typeof(objNodenodeType) == "undefined") { alert("Invalid node -- no nodeType property!"); return; } if (objNodenodeType < 1 || objNodenodeType > 12) { alert("Invalid node -- nodeType is invalid!"); return; } }
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This version of ASSERT(), named ASSERT_VALID_NODE, accepts a single argument and determines whether the node argument is, in fact, a valid node The first test is whether an object was even specified If the argument is null, you definitely don t have a valid node The next test makes sure that the nodeType property of the object is actually present (note that this test is specifically for JavaScript Java and C++ wouldn t need to do this) If the supplied object has no nodeType property, the node can t be valid Finally, you check to make sure that the nodeType property is inside the range of acceptable node types
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This example is pretty basic; it only determines whether the basic structure of a DOM node is present or not You could get a lot more fancy by adding individual checks for each node type For example, this improved version makes sure that the tagName property is present for Element nodes:
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function ASSERT_VALID_NODE(objNode) { if (objNode == null) { alert("Invalid node -- node is null"); return; } if (typeof(objNodenodeType) == "undefined") { alert("Invalid node -- no nodeType property!"); return; } if (objNodenodeType < 1 || objNodenodeType > 12) { alert("Invalid node -- nodeType is invalid!"); return; } if (objNodenodeType == 1) { if (typeof(objNodetagName) == "undefined") { alert("Invalid Element -- no tagName property!"); return; } } }
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Another example of using ASSERT() is to make sure that your own application logic is running correctly For example, suppose there is a point in your DOM application that requires you to act on the document element node You can make sure of this by using a specialized ASSERT() function such as ASSERT_IS_ DOC_ELEMENT:
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function ASSERT_IS_DOC_ELEMENT(objNode) { if (objNode != documentdocumentElement)
AM FL Y
7: Debugging DOM Applications
alert("ASSERT Failure: this is not the document element!"); }
You get the idea When properly used, simple tools like ASSERT() can really increase the effectiveness of your debugging efforts by forcing bugs to appear sooner in the testing process Using them throughout your code is definitely a good programming habit to develop If you re using a development environment such as Microsoft s Visual Studio, you ve already got ASSERT() at your disposal If not, it s easy to define your own version for your particular environment
Tracing Your Steps with TraceConsole
Although using the ASSERT() function will help find errors in programming logic by exposing illegal conditions in the code, sometimes bugs take on more insidious forms that cannot be detected or caught by an ASSERT() statement To find these bugs, you need a slightly more advanced debugging tool The TraceConsole is a utility for browser-based JavaScript DOM developers that displays a program s variables and other interesting objects as they change while the application is running It essentially provides a record of program execution that can be examined after the program has run to help pinpoint logic errors in the code Figure 7-2 shows a sample screenshot of the TraceConsole Almost every JavaScript developer has used the alert() function at some point in their debugging process; it s almost a rite of passage as a web developer The basic premise is that at some point in your application, you need to verify that a certain variable contains a certain value, or that an array contains the right data The main problem with alert() is that it isn t very flexible First, you have to deal with a pop-up window every time the alert() function is called, which can make using alert() inside loops very unpractical Second, the amount of data that can be reasonably displayed inside an alert() window is pretty small Once you start getting more than a half dozen or so lines of output, the readability of the information rapidly diminishes Third, the information displayed by alert() is fleeting Once you dismiss the dialog box, that information is gone (unless you ve manually written it down, which can become a huge pain) Enter the TraceConsole Think of TraceConsole as alert() on steroids: instead of constantly bugging you with pop-up windows that must be manually dismissed before the execution of the code can continue in the browser, the data that would normally be displayed by the alert() function is routed to a separate window
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