The Attr interface represents an attribute in an
Element object. Typically the allowable values for the
attribute are defined in a document type definition.
Attr objects inherit the Node interface, but
since they are not actually child nodes of the element they describe, the
DOM does not consider them part of the document tree. Thus, the
Node attributes parentNode,
previousSibling, and nextSibling have a
null value for Attr objects. The DOM takes the
view that attributes are properties of elements rather than having a
separate identity from the elements they are associated with; this should
make it more efficient to implement such features as default attributes
associated with all elements of a given type. Furthermore,
Attr nodes may not be immediate children of a
DocumentFragment. However, they can be associated with
Element nodes contained within a
DocumentFragment. In short, users and implementors of the
DOM need to be aware that Attr nodes have some things in
common with other objects inheriting the Node interface, but
they also are quite distinct.
The attribute's effective value is determined as follows: if this
attribute has been explicitly assigned any value, that value is the
attribute's effective value; otherwise, if there is a declaration for
this attribute, and that declaration includes a default value, then that
default value is the attribute's effective value; otherwise, the
attribute does not exist on this element in the structure model until it
has been explicitly added. Note that the nodeValue attribute
on the Attr instance can also be used to retrieve the string
version of the attribute's value(s).
In XML, where the value of an attribute can contain entity references,
the child nodes of the Attr node may be either
Text or EntityReference nodes (when these are
in use; see the description of EntityReference for
discussion). Because the DOM Core is not aware of attribute types, it
treats all attribute values as simple strings, even if the DTD or schema
declares them as having tokenized types.
See also the Document Object Model (DOM) Level 2 Core Specification. |
