_counter.js

function increment(props, state) {
  return {
    value: state.value + props.step,
  };
}

function decrement(props, state) {
  return {
    value: state.value - props.step,
  };
}

function Counter(props: { step: number }, state = { value: 0 }, updater) {
  return (
    <div>
      <button onClick={updater(increment)}>+</button>
      <h1>{state.value}</h1>
      <button onClick={updater(decrement)}>-</button>
    </div>
  );
}

ReactDOM.render(
  <Counter step={5}/>,
  document.getElementById('root')
);

counter-all-modern-like.js

let increment = (props, state) => ({
  value: state.value + props.step,
})

let decrement = (props, state) => ({
  value: state.value - props.step,
})

let Counter = (props: { step: number }, state = { value: 0 }, updater) => (
  <div>
    <button onClick={updater(increment)}>+</button>
    <h1>{state.value}</h1>
    <button onClick={updater(decrement)}>-</button>
  </div>
)

ReactDOM.render(
  <Counter step={5}/>,
  document.getElementById('root')
)