The read-only prefix property of the Attr returns the namespace prefix of the attribute, or null if no prefix is specified.

The prefix is always in lower case, whatever case is used at the attribute creation.

Note: Only XML supports namespaces. HTML does not. That means that the prefix of an attribute of an HTML element will always be null.

Also, only the xml (for the xml:lang attribute), xlink (for the xlink:href, xlink:show, xlink:target and xlink:title attributes) and xpath namespaces are supported, and only on SVG and MathML elements.


A string containing the prefix of the namespace the attribute belongs too. If none, it returns null.


HTML Content

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

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

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

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.prefix;


DOM Standard
# dom-attr-prefix

Browser compatibility

BCD tables only load in the browser

See also