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CHAPTER 3 CONTENT CHUNKING PATTERN the innerHTML property. However, things can run amok if the innerHTML property is manipulated when it should not be manipulated or when doing so will violate the structure of the HTML. For instance, as illustrated in the example you cannot create a table without rows or cells. Another way to interact with the HTML Document Object Model is to use individual elements that are instantiated, manipulated, and deleted. Using the Document Object Model, it is much harder to mess up because this model supports only certain methods and properties. When using the HTML Document Object Model, it is not as simple to arbitrarily remove all the rows and replace them with text. There are no methods on the table object model to create a construct, as illustrated in Figure 3-8. It is important to remember that entire chunks of HTML content are replaced when using the Content Chunking pattern. So even though the property innerHTML is powerful and flexible, replacing the wrong chunk at the wrong time will result in an incorrectly formatted HTML page. What you need to remember is that when referencing HTML elements in the context of the pattern,only framework HTML elements usedto contain content chunks shouldbereferenced. As a pattern rule, script in the HTML framework page should not directly reference injected elements, as that would create a dynamic dependency that may or may not work. If such a dependency is necessary, encapsulate the functionality and call a method on the injected elements. JavaScript allows the assignment of arbitrary functions on HTML elements. Identifying Elements It was previously mentioned that when finding elements by using a tag type, it is not possible E B V N to know the identifier; and when finding elements using the identifier, it is not possible to know the tag type. Regardless of how the elements have been found, they are considered a starting point from which manipulations can happen. Based on the starting point, a script can navigate the parent or the child elements by using a number of standard properties and methods. These standard properties and methods are available on virtually all HTML elements, and script writers should focus on using them when navigating a hierarchy, modifying the look and feel, or attempting to identify what the element is. Table 3-1 outlines properties that are of interest when writing scripts. Table 3-1. HTML Element Properties Useful for Writing Scripts Property Identifier Description attributes[ Contains a read-only collection of the attributes associated with the HTML element. An individual attribute can be retrieved by using the method getAttribute. To assign or overwrite an attribute, the method setAttribute is used. To remove an attribute, the method removeAttribute is used. childNodes[ Is an instance of NodeList that most likely is referenced by using an array notation, but the array is read-only. To add a child node to the current element, the method appendChild is used. To remove a child node, the method removeChild is used; and to replace a child node, replaceChild is used. className Assigns a stylesheet class identifier to an element. A class type is very important in Dynamic HTML in that the look and feel of the element can be dynamically assigned. dir Indicates the direction of the text, either left to right (ltr) or right to left (rtl).
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CHAPTER 3 CONTENT CHUNKING PATTERN Figure 3-8. Modified contents of the table after replacing the rows and cells Figure 3-8 illustrates that the table rows have been replaced with nothing. If the TestTable E B V N button is clicked to validate the state, an error is generated, as illustrated in Figure 3-9. Figure 3-9. Object model exception The exception is important and relates to how the property innerHTML operates. When HTML content is assigned using the innerHTMLproperty, the data is text based. When retrieving the value of the innerHTML property, child elements are converted into a text buffer. Assigning the innerHTML property means replacing the child elements with the HTML text defined by the assignment. Then that new HTML text is converted into a series of HTML elements that are presented to the user. The functions GoodReplace and BadReplace are examples of manipulating
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CHAPTER 3 CONTENT CHUNKING PATTERN Figure 3-6. Displaying the contents of the cell mycell E B V N Figure 3-7. Modified contents of the cell For interest, let s add some complications by clicking the button BadReplace. Clicking BadReplace calls the function BadReplace, and that assigns the property innerHTML of the HTML table with other text. This means that the HTML content

is changed to

. The changed HTML is not legal and is displayed as shown in Figure 3-8.
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CHAPTER 3 CONTENT CHUNKING PATTERN In this example, there are three buttons (GoodReplace, BadReplace, and TestTable), and the HTML elements table, table row, and row cell have added identifiers. The GoodReplace button will perform a legal HTML injection. The BadReplace button will perform an illegal HTML injection. And the TestTable button is used to test the validity of an object model. The TestTable button is used as a way of verifying the result of the HTML injection performed by either GoodReplace or BadReplace. Downloading the HTML page and presenting it in the browser results in something similar to Figure 3-5. E B V N Figure 3-5. Initial generation of the HTML page To check that the HTML page is in a valid state, the button TestTable is clicked. Clicking the button calls the function TestTable, which tests whether the content within a table cell exists by outputting the content in a dialog box. The generated output appears similar to Figure 3-6. The dialog box in Figure 3-6 confirms that the table cell contains the value Nothing. This means our HTML page is in a stable state. If the GoodReplace button is clicked, the function GoodReplace is called, which changes the table cell contents from Nothing to Hello. To verify that the HTML page is still valid, the TestTable button is clicked. If the HTML page is valid, a dialog with the text Hello should appear, and it does, as is illustrated in Figure 3-7.
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CHAPTER 3 CONTENT CHUNKING PATTERN nor accessible because the method getElementById returns only a single HTML element instance. Unlike the getElementsByTagName method, the returned element is not guaranteed to be a certain type other than having the parameter identifier equal to the id attribute. As a result, the object model referenced after the getElementById method may or may not apply to the found element. In the case of the property innerHTML, that is not a problem because virtually all visible elements have the innerHTML property. What could be more problematic is if the identifier assumed the retrieved element were a table when in fact the element is a table cell. At that point, the object model referencing would result in an exception. When writing JavaScript code that dynamically retrieves an HTML element(s), it is a good idea to test the found element before manipulating it. As a rule of thumb, when using getElementsByTag, you know what the HTML elements are but do not know where they are or what they represent. When using getElementById, you know what the found HTML element represents and where it is, but do not know the type and hence the object hierarchy. Understanding the Special Nature of innerHTML The property innerHTML is special in that it seems simple to use but can have devastating consequences. To illustrate the problem, consider the following HTML code: