JavaScript Document xmlVersion Property

What You’ll Learn

Document.xmlVersion is a deprecated read-only instance property that reported the version from the XML declaration. Learn why it almost always returned "1.0", how it pairs with xmlEncoding, MDN’s HTML-vs-XML detection trick, and five examples with try-it labs.

Introduction

XML documents can begin with a declaration such as <?xml version="1.0"?>. The version attribute tells parsers which XML specification rules apply.

MDN: Document.xmlVersion returned that version number, or "1.0" when the declaration was absent. MDN also warns the property was never really useful—it almost always returned "1.0"—and it was removed from DOM Level 4. Firefox 10 no longer implements it.

Related Document tutorials: xmlEncoding, compatMode, contentType, Document constructor.

Understanding Document.xmlVersion

A read-only instance property from the legacy DOM XML era.

• Value — version string from XML declaration (MDN), e.g. "1.0".
• Default — "1.0" when the declaration is absent (MDN).
• Low utility — MDN: almost always returned "1.0".
• Read-only — you cannot assign a new version here.
• Deprecated — removed from DOM Level 4; not in Firefox 10+ (MDN).

📝 Syntax

document.xmlVersion

Value

A string (MDN). Typically "1.0".

MDN XML declaration example

<?xml version="1.0"?>
<!-- document.xmlVersion would be "1.0" on legacy XML docs -->

⚖️ xmlVersion vs xmlEncoding vs contentType

Examples Gallery

Examples follow MDN Document: xmlVersion. On HTML try-it pages, the property may be missing (Firefox) or return "1.0" in some legacy engines.

console.log(document.xmlVersion);
// May be undefined (Firefox 10+) or "1.0" in some browsers

undefined  // Firefox 10+
"1.0"      // some legacy engines

<?xml version="1.0"?>
<root/>

// Legacy XML document (MDN):
console.log(document.xmlVersion); // "1.0"

1.0

console.log({
  xmlVersion: document.xmlVersion,
  xmlEncoding: document.xmlEncoding
});
// Typical HTML: { xmlVersion: "1.0" | undefined, xmlEncoding: null }

{ xmlVersion: undefined, xmlEncoding: null }

if (document.createElement("foo").tagName === "FOO") {
  console.log("Document is in non-XML HTML mode");
} else {
  console.log("Document is in XML mode");
}

Document is in non-XML HTML mode

function isHtmlDocument(doc) {
  return doc.createElement("foo").tagName === "FOO";
}

console.log({
  isHtml: isHtmlDocument(document),
  contentType: document.contentType
});

{ isHtml: true, contentType: "text/html" }

🚀 Why This Property Existed

• Legacy XML documents — read declaration version from the DOM (historical).
• HTML vs XML sniffing — replaced by MDN’s createElement tagName test.
• Migration audits — find xmlVersion reads and remove them.
• Old tutorials — understand pre-DOM4 XML Document APIs.
• Not informative — MDN: almost always returned "1.0".
• Firefox note — removed since Firefox 10.0 (MDN).

📝 Notes

• MDN: Deprecated — do not use; removed from DOM Level 4.
• Firefox: not supported since version 10.0 (MDN warning).
• Returns "1.0" when XML declaration is absent (MDN).
• MDN: property was never really useful—almost always "1.0".
• HTML vs XML: use createElement("foo").tagName === "FOO" (MDN).
• Related: xmlEncoding, compatMode, contentType.

💡 Best Practices