Add useState example

examples/useStateAndUseEffect/ClickCounter.test.tsx

@@ -0,0 +1,28 @@
+import React from 'react';
+import { render, screen, fireEvent } from '@testing-library/react';
+
+import { ClickCounter } from './ClickCounter';
+
+describe('Test ClickCounter', () => {
+  it('should inc/dec counter when buttons inc/dec clicked', () => {
+    render(<ClickCounter />);
+
+    const counter = screen.getByRole('heading');
+    const increaseBtn = screen.getByRole('button', { name: 'Increase' });
+    const decreaseBtn = screen.getByRole('button', { name: 'Decrease' });
+
+    expect(counter.textContent).toBe('Counter: 0');
+
+    fireEvent.click(increaseBtn);
+    expect(counter.textContent).toBe('Counter: 1');
+
+    fireEvent.click(increaseBtn);
+    expect(counter.textContent).toBe('Counter: 2');
+
+    fireEvent.click(decreaseBtn);
+    expect(counter.textContent).toBe('Counter: 1');
+
+    fireEvent.click(decreaseBtn);
+    expect(counter.textContent).toBe('Counter: 0');
+  });
+});

examples/useStateAndUseEffect/ClickCounter.tsx

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+import React, { FC, useState } from 'react';
+
+interface Props {
+  defaultCount?: number;
+}
+
+export const ClickCounter: FC<Props> = ({ defaultCount = 0 }) => {
+  const [count, setCount] = useState(defaultCount);
+
+  return (
+    <div>
+      <h1>Counter: {count}</h1>
+      <button onClick={() => setCount(count + 1)}>Increase</button>
+      <button onClick={() => setCount(count - 1)}>Decrease</button>
+    </div>
+  );
+};