Syntax

isDefaultNamespace(namespaceURI)

Parameters

namespaceURI

A string representing the namespace against which the element will be checked. The empty string is equivalent to null.

Note: namespaceURI is not an optional parameter, but can be null.

Return value

A boolean value that holds the return value true or false, indicating if the parameter is the default namespace, or not. It is equivalent to node.lookupNamespaceURI(null) === namespaceURI.

Note: This example runs in an HTML document, where xmlns: attributes are ignored (except xmlns:xlink). Firefox sets all elements' namespace URI to null, while Chrome and Safari appropriately set HTML, SVG, and MathML elements' default namespace URIs. If you want to conduct more meaningful tests, you can open a standalone SVG document and execute scripts in its context.

html

<div class="hidden">
  <div>Test HTML element</div>
  <svg>
    <text>Test SVG element</text>
  </svg>
  <svg xmlns:xlink="http://www.w3.org/1999/xlink" id="with-xlink">
    <text>Test SVG element with xlink</text>
  </svg>
  <math>Test MathML element</math>
</div>
<table>
  <thead>
    <tr>
      <th><code>namespaceURI</code></th>
      <th><code>&lt;div&gt;</code></th>
      <th><code>&lt;svg&gt;</code></th>
      <th><code>&lt;svg xmlns:xlink&gt;</code></th>
      <th><code>&lt;math&gt;</code></th>
    </tr>
  </thead>
  <tbody></tbody>
</table>

.hidden {
  display: none;
}

const htmlElt = document.querySelector("div");
const svgElt = document.querySelector("svg");
const svgEltXLink = document.querySelector("#with-xlink");
const mathElt = document.querySelector("math");
const tbody = document.querySelector("tbody");
for (const uri of [
  "http://www.w3.org/2000/xmlns/",
  "http://www.w3.org/XML/1998/namespace",
  "http://www.w3.org/1999/xhtml",
  "http://www.w3.org/2000/svg",
  "http://www.w3.org/1999/xlink",
  "http://www.w3.org/1998/Math/MathML",
  "",
  null,
]) {
  const row = document.createElement("tr");
  tbody.appendChild(row);
  row.appendChild(document.createElement("td")).textContent =
    JSON.stringify(uri);
  for (const el of [htmlElt, svgElt, svgEltXLink, mathElt]) {
    console.log(el, uri, el.isDefaultNamespace(uri));
    row.appendChild(document.createElement("td")).textContent = String(
      el.isDefaultNamespace(uri),
    );
  }
}

Specifications

Specification
DOM # dom-node-isdefaultnamespace

Browser compatibility

Help improve MDN

Yes No

Learn how to contribute

This page was last modified on Oct 23, 2025 by MDN contributors.