7.4. XML Document Object Model

The majority of web developers are familiar with the HTML DOM, but unless they're used to XML development, they might not even realize that the XML DOM exists. In fact, even if they are aware that there is a Document Object Model for use with XML, they might not know that there is a difference between the XML and the HTML DOM. For example, the HTML DOM is geared more toward the various HTML elements, whereas the XML Document Object Model is somewhat more generic.

The XML Document Object Model is a common API for dealing with XML. It provides a standard interface for accessing, modifying, and creating the various parts of an XML document. Let's take a look at the XML document shown in Listing 7-3 as a starting point, and you'll see what I mean.

Listing 7-3. An XML Document

<?xml version="1.0"?> <garden> <plant> <name>Foxglove</name> <use>heart</use> <part>root</part> </plant> <plant> <name>Mandrake</name> <use>impotency</use> <part>root</part> </plant> <plant> <name>Trillium</name> <use>poison</use> <part>leave</part> </plant> <plant> <name>Wolfsbane</name> <use>werewolf repellent</use> <part>flower</part> </plant> <plant> <name>Meadowsweet</name> <use>cramps</use> <part>leave</part> </plant> </garden>

After the requisite browser-specific JavaScript is executed and the XML document from Listing 7-3 is loaded into a variablesay, myXMLit is time to try out the DOM. Let's say, for instance, that we're interested in getting all the plant nodes in a node set. Using the DOM, we could code the following:

var myNodeset = myXML.getElementsByTagName('plant');

Pretty slick, isn't it?

But there's more to the XML Document Object Model than the getElementsByTagName method. In fact, an entire slew of properties and methods is available by using the XML DOM interfaces in JavaScript. However, to use these properties and methods, it is necessary to know the various interfaces available in JavaScript, as outlined in Table 7-2.

Table 7-2. JavaScript Interfaces Relevant to Using the XML DOM

Interface Name	Description
DOMException	Exception raised by a DOM method when the requested action cannot be completed
ExceptionCode	Integer that indicates the type of error raised by a DOMException
DOMImplementation	Provides methods that are independent of any implementation of the XML Document Object Model
DocumentFragment	A lightweight XML document, often used to hold portions of an XML document
Document	Used to hold an entire XML document
Node	Represents a single node of an XML document
NodeList	An indexed list of nodes
NamedNodeMap	A collection of nodes that are accessed either by name or by index
CharacterData	Extends the Node interface by adding characterspecific properties and methods
Attr	Represents the attributes for individual elements
Element	Extends the Node interface by adding methods for accessing and adding attributes
Text	Represents the text content of an Element
Comment	Represents an XML comment, the text between <!-- and -->
CDATASection	Interface used to escape text that would normally be parsed as XML
DocumentType	Used to define the document type
Notation	Represents a notation declared in the Document Type Definition (DTD)
Entity	Interface used to represent an XML entity, which can be either parsed or unparsed
EntityReference	Interface that contains a reference to an XML entity
ProcessingInstruction	Interface that contains a text-processing instruction

Each of these interfaces has a number of properties and methods that can be used to manipulate an XML document. Table 7-3 lists the various properties and methods, along with their associated interfaces.

Table 7-3. Properties and Methods for Various Interfaces

Property/Method	Interface	Description
hasFeature(feature,version)	DOMImplementation	Returns a Boolean indicating whether the feature is supported.
Doctype	Document	The DTD associated with this XML document.
Implementation	Document	The DOMImplementation for this document.
documentElement	Document	The document's root element.
createElement(tagName)	Document	Creates the specified element.
createDocumentFragment()	Document	Creates an empty document fragment.
createTextNode(data)	Document	Creates a Text element using the data provided.
createComment(data)	Document	Creates a Comment node using the data provided.
createCDATASection(data)	Document	Creates a CDATASection node using the data provided.
createProcessingInstruction (target,data)	Document	Creates a ProcessingInstruction node.
createAttribute(name)	Document	Creates an Attribute.
createEntityReference(name)	Document	Creates an EntityReference.
getElementsByTagName (tagname)	Document Node CharacterData Attr Element Text Comment CDATASection DocumentType Notation Entity EntityReference ProcessingInstruction	Returns a node set consisting of elements with matching tag names.
nodeName	Document Node CharacterData Attr Element Text Comment CDATASection DocumentType Notation Entity EntityReference ProcessingInstruction	The name of the node.
nodeValue	Document Node CharacterData Attr Element Text Comment CDATASection DocumentType Notation Entity EntityReference ProcessingInstruction	The value of the node.
nodeType	Document Node CharacterData Attr Element Text Comment CDATASection DocumentType Notation Entity EntityReference ProcessingInstruction	The type of the node. See Table 7-4 for accepted values.
parentNode	Document Node CharacterData Attr Element Text Comment CDATASection DocumentType Notation Entity EntityReference ProcessingInstruction	The parent of the current node.
childNodes	Document Node CharacterData Attr Element Text Comment CDATASection DocumentType Notation Entity EntityReference ProcessingInstruction	A node set consisting of the child nodes of the current node. Note that the node set may be empty.
firstChild	Document Node CharacterData Attr Element Text Comment CDATASection DocumentType Notation Entity EntityReference ProcessingInstruction	The first child node of the current node.
lastChild	Document Node CharacterData Attr Element Text Comment CDATASection DocumentType Notation Entity EntityReference ProcessingInstruction	The last child of the current node.
previousSibling	Document Node CharacterData Attr Element Text Comment CDATASection DocumentType Notation Entity EntityReference ProcessingInstruction	The previous child of the current node's parent.
nextSibling	Document Node CharacterData Attr Element Text Comment CDATASection DocumentType Notation Entity EntityReference ProcessingInstruction	The next child of the current node's parent.
Attributes	Document Node CharacterData Attr Element Text Comment CDATASection DocumentType Notation Entity EntityReference ProcessingInstruction	A collection consisting of the attributes for the current node.
ownerDocument	Document Node CharacterData Attr Element Text Comment CDATASection DocumentType Notation Entity EntityReference ProcessingInstruction	The Document associated with the current element.
insertBefore(new,reference)	Document Node the CharacterData Attr Element Text Comment CDATASection DocumentType Notation Entity EntityReference ProcessingInstruction	Inserts the new child node before reference child node.
replaceChild(new,old)	Document Node CharacterData Attr Element Text Comment CDATASection DocumentType Notation Entity EntityReference ProcessingInstruction	Replaces the old child node with the new child node.
removeChild(old)	Document Node CharacterData Attr Element Text Comment CDATASection DocumentType Notation Entity EntityReference ProcessingInstruction	Removes the old child node.
appendChild(new)	Document Node CharacterData Attr Element Text Comment CDATASection DocumentType Notation Entity EntityReference ProcessingInstruction	Appends the new child node as the last child.
hasChildNodes()	Document Node CharacterData Attr Element Text Comment CDATASection DocumentType Notation Entity EntityReference ProcessingInstruction	Returns a true if child nodes exist and a false if child nodes do not exist.
cloneNode(deep)	Document Node CharacterData Attr Element Text Comment CDATASection DocumentType Notation Entity EntityReference ProcessingInstruction	Duplicates the specified node. The Boolean parameter deep is used to indicate a deep copy, which states whether the children should be copied.
Length	NodeList NamedNodeList CharacterData	The number of items in the collection or the length of the character data.
item(index)	NodeList NamedNodeList	Returns a single node from a collection based upon the index.
getNamedItem(name)	NamedNodeMap	Returns a single node based upon the node name.
setNamedItem(node)	NamedNodeMap	Adds a single node.
removeNamedItem(name)	NamedNodeMap	Removes a node based upon the node name.
Data	CharacterData Text Comment CDATASection ProcessingInstruction	The character data for the node.
substringData(offset,length)	CharacterData Text Comment CDATASection	Extracts a substring from the character data for the node.
appendData(string)	CharacterData Text Comment CDATASection	Appends the string to the end of the node's character data.
insertData(offset,string)	CharacterData Text Comment CDATASection	Inserts the string into the node's character data at the offset.
deleteData(offset,length)	CharacterData Text Comment CDATASection	Deletes the number of characters specified by the length, starting at the offset.
replaceData (offset,length,string)	CharacterData Text Comment CDATASection	Replaces the number of characters specified by the length, starting at the offset with the specified string.
Name	Attr DocumentType	The attribute name or the DTD name, in the case of the DocumentType.
Specified	Attr	A Boolean indicating whether the attribute has a value in the original document.
Value	Attr	The string value of the attribute.
tagName	Element	The tag name of the Element.
getAttribute(name)	Element	Returns the value of an attribute based upon name.
setAttribute(name,value)	Element	Creates an attribute and sets its value.
removeAttribute(name)	Element	Removes an attribute by name.
getAttributeNode(name)	Element	Retrieves an Attr node by name.
setAttributeNode(name)	Element	Adds an Attr node by name.
removeAttributeNode(name)	Element	Removes an Attr node by name.
normalize()	Element	Normalizes the specified element and children of the specified element.
splitText(offset)	Text	Splits the Text node into two Text nodes at the specified offset.
Entities	DocumentType	A NamedNodeMap containing the entities declared in the DTD.
Notations	DocumentType	A NamedNodeMap containing the notations declared in the DTD.
publicId	Notation Entity	The public identifier for this notation, or a null if no public identifier is specified.
systemId	Notation Entity	The system identifier for this notation, or a null if no system identifier is specified.
notationName	Entity	The name of the notation for this entity if the entity is unparsed. If the entity is parsed, the result is null.
Target	ProcessingInstruction	The target for this processing instruction.

Table 7-4. The Node Types

Node Type	Value	Interface
ELEMENT_NODE	1	Element
ATTRIBUTE_NODE	2	Attr
TEXT_NODE	3	Text
CDATA_SECTION_NODE	4	CDATASection
ENTITY_REFERENCE_NODE	5	EntityReference
ENTITY_NODE	6	Entity
PROCESSING_INSTRUCTION_NODE	7	ProcessingInstruction
COMMENT_NODE	8	Comment
DOCUMENT_NODE	9	Document
DOCUMENT_TYPE_NODE	10	DocumentType
DOCUMENT_FRAGMENT_NODE	11	DocumentFragment
NOTATION_NODE	12	Notation

By using these interfaces, it is possible to manipulate an XML document without really having to mess around too much. The only real issue is the vast array of properties and methods available. They can be rather overwhelming. But personally, I find myself using a narrow range of properties and methods to perform any task that is needed. This narrow range includes methods such as getElementsByTagname and attributes.