Node.namespaceURI

The Node.namespaceURI read-only property returns the namespace URI of the node, or null if the node is not in a namespace. When the node is a document, it returns the XML namespace for the current document.

namespace = node.namespaceURI

In this snippet, a node is being examined for its Node.localName and its namespaceURI. If the namespaceURI returns the XUL namespace and the localName returns "browser", then the node is understood to be a XUL <browser/>.

if (node.localName == "browser" &&
    node.namespaceURI == "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul") {
  // this is a XUL browser
}

This is not a computed value that is the result of a namespace lookup based on an examination of the namespace declarations in scope. The namespace URI of a node is frozen at the node creation time.

In Firefox 3.5 and earlier, the namespace URI for HTML elements in HTML documents is null. In later versions, in compliance with HTML5, it is https://www.w3.org/1999/xhtml as in XHTML.

For nodes of any Node.nodeType other than ELEMENT_NODE and ATTRIBUTE_NODE the value of namespaceURI is always null.

You can create an element with the specified namespaceURI using the DOM Level 2 method Document.createElementNS and attributes with the method Element.setAttributeNS.

Per the Namespaces in XML specification, an attribute does not inherit its namespace from the element it is attached to. If an attribute is not explicitly given a namespace, it has no namespace.

The DOM does not handle or enforce namespace validation per se. It is up to the DOM application to do any validation necessary. Note too that the namespace prefix, once it is associated with a particular node, cannot be changed.

• Node.localName
• Node.prefix
• Element.namespaceURI
• Attr.namespaceURI