Add tests for useImmediateState

src/hooks/useImmediateState.test.ts

@@ -0,0 +1,46 @@
+import React from 'react';
+import { renderHook } from 'react-hooks-testing-library';
+import { useImmediateState } from './useImmediateState';
+
+it('updates state immediately during mount', () => {
+  let initialState;
+  let update = 0;
+
+  const setState = jest.fn();
+
+  const spy = jest.spyOn(React, 'useState').mockImplementation(state => {
+    expect(state).toEqual({ x: 'x', test: false });
+    initialState = state;
+    return [state, setState];
+  });
+
+  const { result } = renderHook(() => {
+    const [state, setState] = useImmediateState({ x: 'x', test: false });
+    if (update === 0) setState(s => ({ ...s, test: true }));
+    update++;
+    return state;
+  });
+
+  expect(setState).not.toHaveBeenCalled();
+  expect(result.current).toEqual({ x: 'x', test: true });
+  expect(result.current).toBe(initialState);
+  expect(update).toBe(1);
+
+  spy.mockRestore();
+});
+
+it('behaves like useState otherwise', () => {
+  const setState = jest.fn();
+  const spy = jest
+    .spyOn(React, 'useState')
+    .mockImplementation(state => [state, setState]);
+
+  renderHook(() => {
+    const [state, setState] = useImmediateState({ x: 'x' });
+    React.useEffect(() => setState({ x: 'y' }), [setState]);
+    return state;
+  });
+
+  expect(setState).toHaveBeenCalledTimes(1);
+  spy.mockRestore();
+});