fix(dom): Preserve default namespace when serializing (#321)

Co-authored-by: Christian Bewernitz <coder@karfau.de>
(cherry picked from commit 8b33c19)

lib/dom.js

@@ -488,6 +488,20 @@ Node.prototype = {
     hasAttributes:function(){
     	return this.attributes.length>0;
     },
+	/**
+	 * Look up the prefix associated to the given namespace URI, starting from this node.
+	 * **The default namespace declarations are ignored by this method.**
+	 * See Namespace Prefix Lookup for details on the algorithm used by this method.
+	 *
+	 * _Note: The implementation seems to be incomplete when compared to the algorithm described in the specs._
+	 *
+	 * @param {string | null} namespaceURI
+	 * @returns {string | null}
+	 * @see https://www.w3.org/TR/DOM-Level-3-Core/core.html#Node3-lookupNamespacePrefix
+	 * @see https://www.w3.org/TR/DOM-Level-3-Core/namespaces-algorithms.html#lookupNamespacePrefixAlgo
+	 * @see https://dom.spec.whatwg.org/#dom-node-lookupprefix
+	 * @see https://github.com/xmldom/xmldom/issues/322
+	 */
     lookupPrefix:function(namespaceURI){
     	var el = this;
     	while(el){
@@ -1175,12 +1189,23 @@ function serializeToString(node,buf,isHTML,nodeFilter,visibleNamespaces){
 		var prefixedNodeName = nodeName
 		if (!isHTML && !node.prefix && node.namespaceURI) {
 			var defaultNS
+			// lookup current default ns from `xmlns` attribute
 			for (var ai = 0; ai < attrs.length; ai++) {
 				if (attrs.item(ai).name === 'xmlns') {
 					defaultNS = attrs.item(ai).value
 					break
 				}
 			}
+			if (!defaultNS) {
+				// lookup current default ns in visibleNamespaces
+				for (var nsi = visibleNamespaces.length - 1; nsi >= 0; nsi--) {
+					var namespace = visibleNamespaces[nsi]
+					if (namespace.prefix === '' && namespace.namespace === node.namespaceURI) {
+						defaultNS = namespace.namespace
+						break
+					}
+				}
+			}
 			if (defaultNS !== node.namespaceURI) {
 				for (var nsi = visibleNamespaces.length - 1; nsi >= 0; nsi--) {
 					var namespace = visibleNamespaces[nsi]

test/dom/serializer.test.js

@@ -119,6 +119,113 @@ describe('XML Serializer', () => {
 			)
 		})
 	})
+	describe('is insensitive to namespace order', () => {
+		it('should preserve prefixes for inner elements and attributes', () => {
+			const NS = 'http://www.w3.org/test'
+			const xml = `
+<xml xmlns="${NS}">
+	<one attr="first"/>
+	<group xmlns:inner="${NS}">
+		<two attr="second"/>
+		<inner:three inner:attr="second"/>
+	</group>
+</xml>
+`.trim()
+			const dom = new DOMParser().parseFromString(xml, 'text/xml')
+			const doc = dom.documentElement
+			const one = doc.childNodes.item(1)
+			expect(one).toMatchObject({
+				localName: 'one',
+				nodeName: 'one',
+				prefix: null,
+				namespaceURI: NS,
+			})
+			const group = doc.childNodes.item(3)
+			expect(group).toMatchObject({
+				localName: 'group',
+				nodeName: 'group',
+				prefix: null,
+				namespaceURI: NS,
+			})
+			const two = group.childNodes.item(1)
+			expect(two).toMatchObject({
+				localName: 'two',
+				nodeName: 'two',
+				prefix: null,
+				namespaceURI: NS,
+			})
+			const three = group.childNodes.item(3)
+			expect(three).toMatchObject({
+				localName: 'three',
+				nodeName: 'inner:three',
+				prefix: 'inner',
+				namespaceURI: NS,
+			})
+			expect(new XMLSerializer().serializeToString(dom)).toEqual(xml)
+		})
+		it('should preserve missing prefixes for inner prefixed elements and attributes', () => {
+			const NS = 'http://www.w3.org/test'
+			const xml = `
+<xml xmlns:inner="${NS}">
+	<inner:one attr="first"/>
+	<inner:group xmlns="${NS}">
+		<inner:two attr="second"/>
+		<three attr="second"/>
+	</inner:group>
+</xml>
+`.trim()
+			const dom = new DOMParser().parseFromString(xml, 'text/xml')
+			const doc = dom.documentElement
+			const one = doc.childNodes.item(1)
+			expect(one).toMatchObject({
+				localName: 'one',
+				nodeName: 'inner:one',
+				prefix: 'inner',
+				namespaceURI: NS,
+			})
+			const group = doc.childNodes.item(3)
+			expect(group).toMatchObject({
+				localName: 'group',
+				nodeName: 'inner:group',
+				prefix: 'inner',
+				namespaceURI: NS,
+			})
+			const two = group.childNodes.item(1)
+			expect(two).toMatchObject({
+				localName: 'two',
+				nodeName: 'inner:two',
+				prefix: 'inner',
+				namespaceURI: NS,
+			})
+			const three = group.childNodes.item(3)
+			expect(three).toMatchObject({
+				localName: 'three',
+				nodeName: 'three',
+				prefix: null,
+				namespaceURI: NS,
+			})
+			expect(new XMLSerializer().serializeToString(dom)).toEqual(xml)
+		})
+		it('should produce unprefixed svg elements when prefixed namespace comes first', () => {
+			const svg = `
+<svg xmlns:svg="http://www.w3.org/2000/svg" xmlns="http://www.w3.org/2000/svg">
+	<g><circle/></g>
+</svg>`.trim()
+			const dom = new DOMParser().parseFromString(svg, 'text/xml')
+
+			expect(new XMLSerializer().serializeToString(dom)).toEqual(svg)
+		})
+		it('should produce unprefixed svg elements when default namespace comes first', () => {
+			const svg = `
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:svg="http://www.w3.org/2000/svg">
+	<g><circle/></g>
+</svg>
+`.trim()
+			const dom = new DOMParser().parseFromString(svg, 'text/xml')
+
+			expect(new XMLSerializer().serializeToString(dom)).toEqual(svg)
+		})
+	})
 	describe('properly escapes attribute values', () => {
 		it('should escape special characters in namespace attributes', () => {
 			const input = `<xml xmlns='<&"' xmlns:attr='"&<'><test attr:test=""/></xml>`