Commented React

//---Component{}... {
class Square extends React.Component
{
  //---constructor...

  constructor() //---set constructor() in Square{ here(){} }
  //---constructor()... {
  {
    //---super(); in constructor
    super();
    //---super(); in constructor

    this.state = {
      value: null,
    };
  } //---constructor()... }

  //---render...

  render() //---set render() in Square{ here(){} }
  //---render()... {
  {
    //---set return() in render(){ here() }
    //---return()... (
    return (
              <button //---set button in render(){ return( <here>...</here> ) }
              //---<button>...

                className="square"


                onClick={
                //---set onClick func in render(){ return( <button here={} />  ) }
                //---onClick=... {
                          () => this.setState(
                                              //---setState() in render(){ return( <button onClick={ here() }> ) }
                                              //---setState()... (
                                              { //---set prop in render(){ return( <button onClick={ setState( { here: } ) }> ) }
                                                //---prop: {
                                                value: 'X'
                                              } //---value }
                                              //---setState()... )
                                             )

                        } //---onClick... }
              > //---inside button element ...
                {this.state.value}
                //---this.state.value = 'X' //a string

                //---button... />
              </button>

            ); //---return()... );


  }//---render()... }
}//---Component{}... }

/////// @mrdignitty
///////
// For passing a func to setState() visit the following link:
//
//link// https://medium.freecodecamp.com/functional-setstate-is-the-future-of-react-374f30401b6b
//
// To better understand how .this works in ES6 visit the following link:
//
//link// https://derickbailey.com/2015/09/28/do-es6-arrow-functions-really-solve-this-in-javascript/
//
// Enjoy your day !