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Dojo widgets, which we ve already discussed in Part II, are certainly part of the DOM But what about DOM elements that aren t associated with widgets They require some attention also Let s do a quick review to remind us what the DOM is It stands for Document Object Model and is the browser s internal representation of the web page We think of a browser as a piece of software that takes an HTML file and displays the page to the computer monitorThis is known as rendering in display terminology However, that understanding of the browser is actually not quite correct What a browser really does is take an HTML file and convert it into an internal representation of the file called a Document Object Model and then renders the DOM Each HTML tag is converted to one or more DOM elementsThis may sound like a distinction without a difference, but it is actually quite important in the Ajax world After the DOM is built, it doesn t have to be staticWe can manipulate elements in the DOM, and the browser will instantly rerender the DOM and change the display that the user
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sees Now, not all DOM changes necessarily cause the display to change, but that is most typical Because Ajax applications rely so heavily on manipulating the DOM, it is important for developers to be able to identify DOM elements and to manipulate them It is possible to do that using JavaScript alone, but we ll use Dojo to make it easier Let s talk first about how to find DOM elements
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The DOM for a typical page can easily contain hundreds and sometimes even thousands of elements, also known as element nodes or just plain nodes If we need to perform some operation on a subset of elements, we need a way to quickly identify those elementsThe DOM provides us a technique for iterating through itself that involves getting all the child nodes for each node (beginning with the root node) and looping through them When a node also has children, we can iterate through those as well Eventually we could walk through the entire DOM tree, testing each element node for whatever properties we are looking forThis brute force method isn t very elegant and doesn t perform very well Are there any alternatives One technique is to provide direct access to a DOM element by specifying its id within the HTML as shown below here:
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<div id="target"></div>
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We can then use the documentgetElementById("target") function to reference the specific DOM element However, this technique is limited because we can only find a single elementWhat about when we want to find a group of elements that possess some common property There is another DOM method available to us, documentgetElementsByTag(), which returns an array of the elements for a specific HTML tag such as <p> for paragraph elements But this function is limited to only allowing us to specify a tagWhat if we want all the elements that use a particular CSS style Cascading Style Sheets (CSS) already provide a method for finding DOM elements by using a technique it calls selectors Selectors are strings that identify DOM elements that styles should be applied toThe selector syntax is very rich and can be used to find elements based on a variety of properties Following is an example of a very simple selector that might be part of your CSS style sheet:
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h1 {color: blue}
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This rule finds all the DOM elements for the <h1> tags and sets their color style property to blue, making the text within the element blueThe h1 part of the rule is the selector, which tells the browser which set of DOM elements that the rule applies to Selectors are very powerful, but they can only be used when applying styles in CSS Wouldn t it be nice if we could somehow use the CSS selector syntax to retrieve a list of elements to be used for other purposes Yes, it would be nice, but that is not part of the JavaScript language However, it turns out that Dojo can give us that capability
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