packages/happy-dom/src/nodes/element/NamedNodeMap.ts

@@ -84,6 +84,7 @@ export default class NamedNodeMap {
 	 * @returns Item.
 	 */
 	public getNamedItem(name: string): Attr | null {
+		name = String(name);
 		if (
 			this[PropertySymbol.ownerElement][PropertySymbol.namespaceURI] === NamespaceURI.html &&
 			this[PropertySymbol.ownerElement][PropertySymbol.ownerDocument][

packages/happy-dom/test/nodes/element/Element.test.ts

@@ -1646,6 +1646,12 @@ describe('Element', () => {
 			element.removeAttribute('key1');
 			expect(element.attributes.length).toBe(0);
 		});
+
+		it('Should stringify a non string attribute and remove it', () => {
+			element.setAttribute('undefined', 'value1');
+			element.removeAttribute(undefined);
+			expect(element.attributes.length).toBe(0);
+		});
 	});
 
 	describe('removeAttributeNS()', () => {

packages/happy-dom/test/nodes/element/NamedNodeMap.test.ts

@@ -129,6 +129,11 @@ describe('NamedNodeMap', () => {
 			expect(element.getAttribute('key')).toBe('value1');
 			expect(element.getAttributeNS('namespace', 'key')).toBe('value2');
 		});
+
+		it('Handles non string keys as strings', () => {
+			element.setAttribute('undefined', 'value1');
+			expect(element.getAttribute(undefined)).toBe('value1');
+		});
 	});
 
 	describe('setNamedItemNS()', () => {