Node: lookupPrefix() method

The lookupPrefix() method of the Node interface returns a string containing the prefix for a given namespace URI, if present, and null if not. When multiple prefixes are possible, the first prefix is returned.

Syntax

lookupPrefix(namespace)

Parameters

• namespace
  • : A string containing the namespace to look the prefix up. The empty string is equivalent to null, both causing null to be returned.

    [!NOTE] This parameter is not optional but can be set to null.

Return value

A string containing the corresponding prefix, or null if none has been found. If namespace is null, or the empty string, lookupPrefix() returns null.

If the node is a DocumentType or a DocumentFragment, lookupPrefix() always returns null.

Example

[!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.

<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.lookupPrefix(uri));
    row.appendChild(document.createElement("td")).textContent = String(
      el.lookupPrefix(uri),
    );
  }
}

Specifications

Browser compatibility