Attr objects represent key-value pairs of attributes on Element objects. Attr objects are immutable.
This is NOT part of any supported API. If you write code that depends on this, you do so at your own risk. This code and its internal interfaces are subject to change or deletion without notice.
The following table gives some examples of the relations between the attribute value in the original document (parsed attribute), the value as exposed in the DOM, and the serialization of the value:
| Examples | Parsed attribute value | Initial Attr.value | Serialized attribute value |
|---|---|---|---|
| Character reference | "x²=5" | "x\u00b2=5" | "x²=5" |
| Built-in character entity | "y<6" | "y<6" | "y<6" |
| Literal newline between | "x=5 y=6" | "x=5 y=6" | "x=5 y=6" |
| Normalized newline between | "x=5 y=6" | "x=5 y=6" | "x=5 y=6" |
Entity e with literal newline | <!ENTITY e '... ...'> [...]> "x=5&e;y=6" | Dependent on Implementation and Load Options | Dependent on Implementation and Load/Save Options |
See also the Document Object Model (DOM) Level 3 Core Specification.
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 provide a representation in which entity references are not expanded. These child nodes may be either Text or EntityReference nodes. Because the attribute type may be unknown, there are no tokenized attribute values.
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