pokemon.trade/app/javascript/packs/frontend/components/test_component.tsx

import React, {useState} from 'react';

function TestComponent(): React.FunctionComponentElement<void> {
  const [text, setText] = useState('');

  return (
    <div>
      <div>
        <h1>React + TypeScript lives!</h1>
      </div>
      Is React working? Test here: {text}
      <hr />
      <input onChange={(e): void => setText(e.target.value)} />
    </div>
  );
}

export default TestComponent;