Welcome to Tic Tac Toe using ReactJS (sin Redux)!
To start the application in a new browser window use:
yarn start
To start and stop the flow server use:
yarn flow-start
yarn flow-stop
To represent an empty square, I used null instead of using a string value. Now that type checking has been added to the application, it would be cleaner to have all the possible square contents values to have the same type, which would fix the one remaining flow type error. [Currently, it is just being suppressed]
Instead of hard-coding the player symbols in the code, it would be better to create a shared class, containing symbol constants for all the possible square contents values.