add test

test/utils.spec.js

@@ -0,0 +1,15 @@
+import { describe, it, expect } from 'vitest';
+import { isString } from '../src/utils.js';
+
+describe('isString', () => {
+  it('should return true for strings', () => {
+    expect(isString('string')).toBe(true);
+  });
+
+  it.each([[undefined], [null], [1], [{}], [[]], [() => {}]])(
+    'should return false for non-strings, testing %o',
+    (value) => {
+      expect(isString(value)).toBe(false);
+    },
+  );
+});