React.Component vs React.PureComponent

I don’t know but many people say “React.PureComponent does shallow comparison over props and state inside shouldComponentUpdate().” But if you test yourself with code like this:

class Parent extends React.Component { <-- not PureComponent
  render() {
    return <div><Child say="hello" /></div>
  }
}

We can see Child never re-render when parent does. So basically, React.Component is already doing shallow comparison over props. So I conclude, React.PureComponent does shallow comparison over state only inside shouldComponentUpdate().

Please correct me if I am wrong.

React.PureComponent is similar to React.Component. The difference between them is that React.Component doesn’t implement shouldComponentUpdate(), but React.PureComponent implements it — React Doc said.