Default implementation of shouldComponentUnpdate

if (type.prototype && type.prototype.isPureReactComponent) {
      return (
        !shallowEqual(oldProps, newProps) || !shallowEqual(oldState, newState)
      );
    }

class Hello extends React.Component {
  render(){
    const {name, handleOnClick} = this.props
    console.log('rendered Hello')
    return (<div>
        hello {name}
        <button onClick={handleOnClick}> Test </button>
      </div>)
  }
}

class App extends React.Component {
          constructor(props){
            super(props);
            const afunction = function(){
              console.log('clicked');
            }
            this.a = afunction;  // yes they are referring to same function
            this.b = afunction;  // yes they are referring to same function
            this.state = {
              selected: this.a
            }
            this.changeSelected = this.changeSelected.bind(this);
          }

          changeSelected() {
            this.setState({
              selected: this.b
            })
          }

          render() {
            console.log('rendered app')
            return(
              <div>
                <Hello name="World" handleOnClick={this.state.selected} />
                <button onClick={this.changeSelected}>Swap function</button>
              </div>
            );

          }
        }