Attr.namespaceURI

The read-only namespaceURI property of the Attr interface returns the namespace URI of the attribute, or null if the element is not in a namespace.

The namespace URI is set at the Attr creation and cannot be changed. An attribute with a namespace can be created using Element.setAttributeNS().

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

Value

A string containing the URI of the namespace, or null if the attribute is not in a namespace.

Example

The following example shows the results for a prefixed attribute in a case of an HTML element, and of a SVG element. As HTML doesn't handle namespaces, it will always return null in that case. In the case of the SVG element, it will return the URI of the XML namespace, http://www.w3.org/XML/1998/namespace.

HTML Content

<svg xml:lang="en-US" class="struct" height="1" width="1">Click me</svg>
<label xml:lang="en-US" class="struct"></label>

<p>
  <button>Show value for &lt;svg&gt;</button>
  <button>Show value for &lt;label&gt;</button>
</p>

<p>
  Namespace URI of the attribute <code>xml:lang</code>:
  <output id="result">None.</output>
</p>

JavaScript Content

const elements = document.querySelectorAll(".struct");
const buttons = document.querySelectorAll("button");
const outputEl = document.querySelector("#result");

let i = 0;
for (const button of buttons) {
  const element = elements[i];
  button.addEventListener("click", () => {
    const attribute = element.attributes[0];
    outputEl.value = attribute.namespaceURI;
  });
  i++;
}

Specifications

Specification
DOM Standard # dom-attr-namespaceuri

Browser compatibility

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