test: add tests for nullishAttributeConverter

src/helpers/nullish-attribute-converter.ts

@@ -1,4 +1,4 @@
 import type { ComplexAttributeConverter } from 'lit';
 
-export const nullishAttributeConverter: ComplexAttributeConverter<unknown>['toAttribute'] =
-  <T = string>(value: T) => value || undefined;
+export const nullishAttributeConverter: NonNullable<ComplexAttributeConverter<unknown>['toAttribute']> =
+  <T = string>(value: T) => value || undefined;

src/tests/helpers/nullish-attribute-converter.test.ts

@@ -0,0 +1,28 @@
+import { expect } from '@open-wc/testing';
+
+import { nullishAttributeConverter } from '../../helpers/nullish-attribute-converter';
+import { messageFormatter } from '../test-utils/message-formatter';
+
+type A = [unknown, string | undefined];
+
+const cases: A[] = [
+  [null, undefined],
+  [undefined, undefined],
+  ['', undefined],
+  ['test', 'test'],
+];
+
+describe(nullishAttributeConverter.name, () => {
+  cases.forEach((a) => {
+    it(
+      messageFormatter('returns normalized attribute value (value=%s)', a),
+      () => {
+        const [testValue, expected] = a;
+
+        const result = nullishAttributeConverter(testValue);
+
+        expect(result).equal(expected);
+      }
+    );
+  });
+});