Add useSetState tests

src/__tests__/useSetState.test.ts

@@ -0,0 +1,42 @@
+import { act, renderHook } from '@testing-library/react-hooks';
+import useSetState from '../useSetState';
+
+const setUp = (initialState?: object) => renderHook(() => useSetState(initialState));
+
+it('should init state and setter', () => {
+  const { result } = setUp({ foo: 'bar' });
+  const [state, setState] = result.current;
+
+  expect(state).toEqual({ foo: 'bar' });
+  expect(setState).toBeInstanceOf(Function);
+});
+
+it('should init empty state if not initial state provided', () => {
+  const { result } = setUp();
+
+  expect(result.current[0]).toEqual({});
+});
+
+it('should merge changes into current state when providing object', () => {
+  const { result } = setUp({ foo: 'bar', count: 1 });
+  const [state, setState] = result.current;
+
+  act(() => {
+    // @ts-ignore
+    setState({ count: state.count + 1, someBool: true });
+  });
+
+  expect(result.current[0]).toEqual({ foo: 'bar', count: 2, someBool: true });
+});
+
+it('should merge changes into current state when providing function', () => {
+  const { result } = setUp({ foo: 'bar', count: 1 });
+  const [, setState] = result.current;
+
+  act(() => {
+    // @ts-ignore
+    setState(prevState => ({ count: prevState.count + 1, someBool: true }));
+  });
+
+  expect(result.current[0]).toEqual({ foo: 'bar', count: 2, someBool: true });
+});