component.tsx

function Calculator() {
  const [input, setInput] = useState('');
  const [result, setResult] = useState<number>()
  return <div>
    <input data-testid='expression' onChange={e => setInput(e.target.value)}/>
    <button data-testid='calculate' onClick={() => setResult(eval(input))}/>
    <div data-testid='result'>{result}</div>
  </div>
};