reducers.js

// Reducer (pure function)
function updatePlayer(player, action) {
  switch (action.type) {
    case 'PICK_SIDE':
      return action.side;
    default:
      return player || '';
  }
}

// Call reducer on Store.update()
Store.prototype.update = function(state, action) {
  return {
    player: updatePlayer(state.player, action)
    // [...other reducers]
  };
};