jQuery and XML
Likely the main reasons for jQuery's huge popularity are its fast and simple traversal engine and its slick selector syntax. (Excellent documentation also really helps.) And although its primary use is HTML processing, in this section you explore how it works and how to apply it to processing XML files as well.
To access any of jQuery's features you first need to make sure that the file jquery.js
is included on the page. Having done that, you simply call
jQuery() or the shorthand version
pass it a selector as the first argument. A selector is usually a string that
specifies an element or a collection of elements if more than an element matches the
given selector. Listing 20 shows some basic jQuery selectors.
Listing 20. Basic jQuery selectors
Listing 21. Basic jQuery operation with chained method calls
This code selects all images, sets padding and border on each of them, then wraps each
in a DIV with class
img-wrap. As you can tell, that's quite a bit of cross-browser
functionality reduced to just a single line of code. For thorough information on
jQuery selectors and methods, check out the excellent documentation on the jQuery
website (see Resources).
Listing 22 shows how jQuery simplifies examples from the previous section.
Listing 22. Creating and injecting a DOM node with jQuery
I mentioned that the first argument passed to the
function is the string selector. The less common second argument allows you to set the
context, or starting node for jQuery, to use as a root when making the selection. By
default, jQuery uses the document element as the context, but optimizing code is possible by restricting the context to a more specific (and therefore smaller) subset of the document. To process XML, you want to set the context to the root XML document (see Listing 23).
Listing 23. Retrieving values from an XML document with jQuery
That code cleans things up quite a bit. By passing the node name to the core
jQuery $() function and setting the context,
xmlData, you quickly get access to the node set you want. Getting the value of the node, though, is something that needs some exploration.
innerHTML property does not work for non-HTML documents, you cannot rely on jQuery's
html() method to retrieve the contents of a node. jQuery also provides a method for cross-browser retrieval of the text of an HTML node. The
text() method, as mentioned earlier, is a cross-browser wrapper for the
innerText property, but even it behaves inconsistently across browsers when processing XML. Internet Explorer, for example, ignores what it considers the empty node values (spaces, tabs, breaks) as the contents of a node. This approach might seem more intuitive than Firefox's handling of the same, which interprets the
related_nodes element from the sample XML file as a set of text nodes along with the
related_items nodes. To get around this inconsistency, create custom methods for treating text nodes consistently. In doing so (see Listing 24) you make use of a few handy jQuery methods:
Now look at how to set the node value (see Listing 25). Two things to keep in mind are that this operation is potentially destructive, as setting the text value of the root node overwrites all of its children. Also note that if a specific node has no prior text value, instead of setting it using
node.textContent, set it with
node["textContent"] because Internet Explorer doesn't like the first method (the property doesn't exist when blank).
Listing 26. Getting and setting DOM element attributes with jQuery
As you can see, jQuery's
attr() method supports both the
retrieval and setting of attributes. More importantly, jQuery provides excellent
access to element retrieval by allowing attributes in selectors. In the example above,
you selected the item with
content_id attribute set to
1, from the
As you probably already know, Ajax is a web technology for asynchronous retrieval of
XMLHttpRequest (XHR) API to send a request to and receive a response
from the server. In addition to providing excellent DOM traversal and manipulation methods, jQuery also offers thorough, cross-browser Ajax support. That said, the loading of XML through Ajax is as native as Ajax gets, so you're on familiar ground. The way this works in jQuery is shown in Listing 27.
Listing 27. Loading an external XML file with jQuery's Ajax method
$.ajax() method has a number of additional options and can also be called indirectly through shortcut methods such as
$.getJSON(), which loads a JSON data file and makes it available to the success script, and so on. When requesting a file of type XML, though, you're stuck with the core
$.ajax() method that has the advantage of forcing you to know only its syntax for any circumstance. In the example above, you simply request file /path/to/data.xml, specifying that the
dataType is "xml" and that the request method is
GET. After the browser receives a response from the server, it triggers either the success or the error callback function accordingly. In this example, a success callback alerts the total number of nodes. jQuery's star selector (*) matches all nodes. The key point is to note that the success callback function receives the data from the server as the first argument. The name of the variable is up to you, and as described earlier, that value becomes the context passed to any jQuery call intended to process the XML.
An important thing to keep in mind when processing Ajax in general is the cross-domain restriction, which prevents retrieval of files from different domains. The previously covered methods of server-side XML retrieval might be viable alternatives in your application.
Because XHTML is a subset of valid XML, there's no reason why you can't process it the same way you process XML. Why exactly you would want to is a separate topic, but the point is that you could. For instance, scrapping a (valid) XHTML page and extracting data from it is perfectly doable using this technique, even though I encourage a more robust approach.
While primarily intended for HTML DOM traversal and manipulation, jQuery can also be used for processing XML as well, though it requires the additional step of a getting the file to the browser. The topics covered in this section explain the different methods and provide the methods essential for processing the XML effectively.