fix(serializer): Serialize void elements as ""

packages/parse5/lib/serializer/index.test.ts

@@ -3,6 +3,7 @@ import * as parse5 from 'parse5';
 import { generateSerializerTests } from 'parse5-test-utils/utils/generate-serializer-tests.js';
 import { treeAdapters } from 'parse5-test-utils/utils/common.js';
 import { type Element, isElementNode } from 'parse5/dist/tree-adapters/default';
+import { NAMESPACES } from 'parse5/dist/common/html.js';
 
 generateSerializerTests('serializer', 'Serializer', parse5.serialize);
 
@@ -42,4 +43,19 @@ describe('serializer', () => {
 
         assert.equal(html, '<button>Hello</button>');
     });
+
+    it('serializes the children of void elements as the empty string (GH-289)', () => {
+        const br = treeAdapters.default.createElement('br', NAMESPACES.HTML, []);
+
+        // Add child node to `br`, to make sure they are skipped.
+        treeAdapters.default.appendChild(br, treeAdapters.default.createElement('div', NAMESPACES.HTML, []));
+
+        assert.equal(parse5.serialize(br), '');
+
+        // If the namespace is not HTML, the serializer should not skip the children.
+        const svgBr = treeAdapters.default.createElement('br', NAMESPACES.SVG, []);
+        treeAdapters.default.appendChild(svgBr, treeAdapters.default.createElement('div', NAMESPACES.HTML, []));
+
+        assert.equal(parse5.serialize(svgBr), '<div></div>');
+    });
 });

packages/parse5/lib/serializer/index.ts

@@ -30,6 +30,15 @@ const VOID_ELEMENTS = new Set<string>([
     $.TRACK,
     $.WBR,
 ]);
+
+function isVoidElement<T extends TreeAdapterTypeMap>(node: T['node'], options: InternalOptions<T>): boolean {
+    return (
+        options.treeAdapter.isElementNode(node) &&
+        options.treeAdapter.getNamespaceURI(node) === NS.HTML &&
+        VOID_ELEMENTS.has(options.treeAdapter.getTagName(node))
+    );
+}
+
 const UNESCAPED_TEXT = new Set<string>([$.STYLE, $.SCRIPT, $.XMP, $.IFRAME, $.NOEMBED, $.NOFRAMES, $.PLAINTEXT]);
 
 export function hasUnescapedText(tn: string, scriptingEnabled: boolean): boolean {
@@ -83,6 +92,11 @@ export function serialize<T extends TreeAdapterTypeMap = DefaultTreeAdapter.Defa
     options?: SerializerOptions<T>
 ): string {
     const opts = { ...defaultOpts, ...options };
+
+    if (isVoidElement(node, opts)) {
+        return '';
+    }
+
     return serializeChildNodes(node, opts);
 }
 
@@ -148,9 +162,7 @@ function serializeElement<T extends TreeAdapterTypeMap>(node: T['element'], opti
     const tn = options.treeAdapter.getTagName(node);
 
     return `<${tn}${serializeAttributes(node, options)}>${
-        options.treeAdapter.getNamespaceURI(node) === NS.HTML && VOID_ELEMENTS.has(tn)
-            ? ''
-            : `${serializeChildNodes(node, options)}</${tn}>`
+        isVoidElement(node, options) ? '' : `${serializeChildNodes(node, options)}</${tn}>`
     }`;
 }