Add a test for asynchronous visitor

.eslintrc.cjs

@@ -2,6 +2,7 @@ module.exports = {
 	env: {
 		node: true,
 		jest: true,
+		es6: true,
 	},
 	extends: [
 		"eslint:recommended",

test/visit.test.mjs

@@ -147,14 +147,34 @@ describe('makeVisitor', () => {
 	});
 
 	it('leaves nodes untouched if transformer returns undefined', async () => {
-		const visitor = makeVisitor([jest.fn(x => undefined)]);
+		const visitor = makeVisitor([jest.fn(() => undefined)]);
 		const result = await visit(mockTree(), visitor);
 		expect(result).toEqual(mockTree());
 	});
 
 	it('throws if a transformer returns strange data', async () => {
-		const visitor = makeVisitor([jest.fn(x => 'foo')]);
+		const visitor = makeVisitor([jest.fn(() => 'foo')]);
 		await expect(visit(mockTree(), visitor))
 			.rejects.toThrow('Invalid transformer result: "foo"');
 	});
+
+	it('awaits visitor functions', async () => {
+		// asyncronously add property to element id=foo
+		const fn = jest.fn(async x => {
+			if (x?.properties?.id === 'foo') {
+				await new Promise(resolve => setTimeout(resolve, 10));
+				return {
+					...x,
+					properties: {
+						...x.properties,
+						awaited: true,
+					},
+				};
+			}
+			return x;
+		});
+		const visitor = makeVisitor([fn]);
+		const result = await visit(mockTree(), visitor);
+		expect(result.children[0].properties.awaited).toBe(true);
+	});
 });