Node and Other Generic Interfaces

The Node interface is the DOM Core class hierarchy's root. Though never instantiated directly, it is the root interface of all specific interfaces, and you can use it to extract information from any DOM object without knowing its actual type. It is possible to access a document's complete structure and content using only the methods and properties exposed by the Node interface. As shown in Table 18-1, this interface contains information about the type, location, name, and value of the corresponding underlying document data.

Table 18-1. Node interface

Type Name Read-only DOM 2.0
Attributes
DOMString nodeName Figure
DOMString nodeValue
Short Unsigned type Figure
Node parentNode Figure
NodeList childNodes Figure
Node firstChild Figure
Node lastChild Figure
Node previousSibling Figure
Node nextSibling Figure
NamedNodeMap attributes Figure
Document ownerDocument Figure Figure
DOMString namespaceURI Figure Figure
DOMString Prefix Figure
DOMString localName Figure Figure
Methods
Boolean hasAttributes Figure
Node insertBefore
Node newChild
Node refChild
Node replaceChild
Node newChild
Node oldChild
Node removeChild
Node oldChild
Node appendChild
Node newChild
Boolean hasChildNodes
Node cloneNode
Boolean Deep
Void normalize Figure
Boolean isSupported Figure
DOMString Feature Figure
DOMString Version Figure

Since the Node interface is never instantiated directly, the nodeType attribute contains a value that indicates the given instance's specific object type. Based on the nodeType, it is possible to cast a generic Node reference safely to a specific interface for further processing. Table 18-2 shows the node type values and their corresponding DOM interfaces, and Table 18-3 shows the values they provide for nodeName, nodeValue, and attributes attributes.

Table 18-2. DOM node types and interfaces

Node type DOM interface
ATTRIBUTE_NODE Attr
CDATA_SECTION_NODE CDATASection
COMMENT_NODE Comment
DOCUMENT_FRAGMENT_NODE DocumentFragment
DOCUMENT_NODE Document
DOCUMENT_TYPE_NODE DocumentType
ELEMENT_NODE Element
ENTITY_NODE Entity
ENTITY_REFERENCE_NODE EntityReference
NOTATION_NODE Notation
PROCESSING_INSTRUCTION_NODE ProcessingInstruction
TEXT_NODE Text

Table 18-3. DOM node types and method results

Node type nodeName nodeValue Attributes
ATTRIBUTE_NODE att name att value null
CDATA_SECTION_NODE #cdata-section content null
COMMENT_NODE #comment content null
DOCUMENT_FRAGMENT_NODE #document-fragment null null
DOCUMENT_NODE #document null null
DOCUMENT_TYPE_NODE document type name null null
ELEMENT_NODE tag name null NamedNodeMap
ENTITY_NODE entity name null null
ENTITY_REFERENCE_NODE name of entity referenced null null
NOTATION_NODE notation name null null
PROCESSING_INSTRUCTION_NODE target content excluding the target null
TEXT_NODE #text content null

Note that the nodeValue attribute returns the contents of simple text and comment nodes, but returns nothing for elements. Retrieving the text of an element requires inspecting the text nodes it contains.

The NodeList Interface

The NodeList interface provides access to the ordered content of a node. Most frequently, it is used to retrieve text nodes and child elements of element nodes. See Table 18-4 for a summary of the NodeList interface.

Table 18-4. NodeList interface

Type Name Read-only DOM 2.0
Attributes
Long length Figure
Methods
Node item
Long index

The NodeList interface is extremely basic and is generally combined with a loop to iterate over the children of a node.

The NamedNodeMap Interface

The NamedNodeMap interface is used for unordered collections whose contents are identified by name. In practice, this interface is used to access attributes. See Table 18-5 for a summary of the NamedNodeMap interface.

Table 18-5. NamedNodeMap interface

Type Name Read-only DOM 2.0
Attributes
Long length Figure
Methods
Node getNamedItem
DOMString name
Node setNamedItem
Node arg
Node removeNamedItem
DOMString name
Node getNamedItemNS Figure
DOMString namespaceURI Figure
DOMString localName Figure
Node setNamedItemNS Figure
Node arg Figure
Node removeNamedItemNS
DOMString namespaceURI Figure
DOMString localName Figure

Relating Document Structure to Nodes

Although the DOM doesn't specify an interface to cause a document to be parsed, it does specify how the document's syntax structures are encoded as DOM objects. A document is stored as a hierarchical tree structure, with each item in the tree linked to its parent, children, and siblings: 

<sample bogus="value"><text_node>Test data.</text_node></sample>

Figure 18-1 shows how the preceding short sample document would be stored by a DOM parser. Figure 18-1

Figure 18-1. Document storage and linkages

Each Node-derived object in a parsed DOM document contains references to its parent, child, and sibling nodes. These references make it possible for applications to enumerate document data using any number of standard tree-traversal algorithms. "Walking the tree" is a common approach to finding information stored in a DOM and is demonstrated in Example 18-1 at the end of this chapter.