onChange & onScoreChange

To answer the first: You can't pass any argument to the function using onClick, so you need to wrap it inside an anonymous function do to so.

Well, more precisely you can't pass your own arguments because onClick passes the its function an Event object with information of the click event. When we write onClick={function() {props.onChange(-1);} we really mean onClick={function(event) {props.onChange(-1);} It's just that the details of the event beyond the button being clicked are irrelevant here so we ignore it. Writing onClick={props.onChange(-1)} is nonsensical (and locks up Firefox with a Unresponsive Script error if you try it).

For your second question: The Player does not receive the arguments. The Application receive them. What the Player does is forwarding the callback from Application (Application.onScoreChange) to the Counter component. When Counter's onChange is trigger, it passes 1 or -1 directly to Application.onScoreChange. Every data here is stored in Application.

i kind of get it, but i think what solved this in my understanding is:

a react expression evaluates before being sent to the onClick property.

so without the anonymous function, props.onChange(1) would evaluate to undefined and be sent like that to onClick. Instead, we need to send the whole function to keep it from trying to execute too early.

This post says it better: http://stackoverflow.com/questions/24050456/onclick-assigned-function-with-parameters

Anyone to answer my second question?