App.js

// in App.js (Parent component)
...
...
...
 // this function is re-created on ever re-execution of this function
  const onDemoClick = () => {
    console.log('Demo clicked!');
  }
  return (
    <div>
      <h2>Virtual DOM demo by Aditya Tyagi</h2>
       {/* because onDemoClick is re-created, the DemoOutput component is re-executed every-time, even though nothing changes in the function */}
      <DemoOutput onClick={onDemoClick} />
      <button onClick={togglePara}>Toggle</button>
    </div>
  );