Element: namespaceURI property

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since ⁨July 2015⁩.

TheElement.namespaceURI read-only property returns the namespace URI of the element, ornull if the element is not in a namespace.

In this snippet, an element is being examined for itslocalName and itsnamespaceURI. If thenamespaceURI returns the XUL namespace and thelocalName returns "browser", then the node is understood to be a XUL<browser/>.

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

Notes

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.

The namespace URI for HTML elements in HTML documents ishttp://www.w3.org/1999/xhtml as in XHTML.

You can create an element with the specifiednamespaceURI using thedocument.createElementNS() method.

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