switch-test-render.js

const Switch = (props) => <span>{props.children}</span>;
const Case = (props) => props.expr && props.children;

const TestInstantiated = (props) => {
  console.log(`${props.id}: I was rendered`);
  return <div>{props.id}</div>;
}

function TestSwitch() {
  const val = 3;
  return (
  <Switch>
    <Case expr={val === 1}>
      <TestInstantiated id="1">One</TestInstantiated>
    </Case>
    <Case expr={val === 2}>
      <TestInstantiated id="2">Two</TestInstantiated>
    </Case>
    <Case expr={val === 3}>
      <TestInstantiated id="3">Three</TestInstantiated>
    </Case>
  </Switch>
  );
}

const elem = document.getElementById('thing'); 
ReactDOM.render(<TestSwitch/>, elem)