reducer.js

const players = (state = [{ id: 1, level: 10 }], action) => {
  if (action.type === 'LEVEL_UP') {
    return state.map(player => {
      if(player.id !== action.id) return state;
      return { ...player, level: player.level + 1  }
	  })
  } else if (action.type === 'LEVEL_DOWN') {
    return state.map(player => {
      if(player.id !== action.id) return state;
      return { ...player, level: player.level - 1  }
    })
  } else {
    return state;
  }
}