Syntax

lookupNamespaceURI(prefix)

Parameters

prefix

The prefix to look for. The empty string is equivalent to null, meaning the default namespace.

Note: This parameter is not optional, but can be set to null.

Return value

A string containing the namespace URI corresponding to the prefix.

Always returns null if the node is a DocumentFragment, DocumentType, Document with no documentElement, or Attr with no associated element.
If prefix is "xml", the return value is always "http://www.w3.org/XML/1998/namespace".
If prefix is "xmlns", the return value is always "http://www.w3.org/2000/xmlns/".
If the prefix is null, the return value is the default namespace URI.
If the prefix is not found, the return value is null.

Note: This example runs in an HTML document, where xmlns: attributes are ignored (except xmlns:xlink). Firefox sets all elements' namespace URIs 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>prefix</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 prefix of ["xmlns", "xml", "html", "svg", "xlink", "", null]) {
  const row = document.createElement("tr");
  tbody.appendChild(row);
  row.appendChild(document.createElement("td")).textContent =
    JSON.stringify(prefix);
  for (const el of [htmlElt, svgElt, svgEltXLink, mathElt]) {
    console.log(el, prefix, el.lookupNamespaceURI(prefix));
    row.appendChild(document.createElement("td")).textContent = String(
      el.lookupNamespaceURI(prefix),
    );
  }
}

Specifications

Specification
DOM # dom-node-lookupnamespaceuri

Browser compatibility

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