R(edux)PG - Run on JSFiddle

main.js

const { createStore, combineReducers } = Redux;

//  The Hero
const initialState = {
  xp: 0,
  level: 1,
  position: {
    x: 0,
    y: 0,
  },
  stats: {
    health: 50,
    maxHealth: 50,
  },
  inventory: {
    potions: 1,
  }
};

//  Actions
const Actions = {
	GAIN_XP: 'GAIN_XP',
  LEVEL_UP: 'LEVEL_UP',
  MOVE: 'MOVE',
  DRINK_POTION: 'DRINK_POTION',
  TAKE_DAMAGE: 'TAKE_DAMAGE',
};

//  Action Creators
const gainXp = (xp) => ({
	type: Actions.GAIN_XP,
  payload: xp
});

const levelUp = () => ({
  type: Actions.LEVEL_UP
});

const move = (x, y) => ({
	type: Actions.MOVE,
  payload: { x, y }
});

const drinkPotion = () => ({
	type: Actions.DRINK_POTION
});

const takeDamage = (amount) => ({
	type: Actions.TAKE_DAMAGE,
  payload: amount
});

//  Reducers
const xpReducer = (state = 0, action) => {
	switch (action.type) {
  	case Actions.GAIN_XP:
    	return state + action.payload;
  }
  return state;
}

const levelReducer = (state = 1, action) => {
  switch (action.type) {
    case Actions.LEVEL_UP:
      return state + 1;
  }
  return state;
};

const positionReducer = (state = initialState.position, action) => {
	switch (action.type) {
		case Actions.MOVE:
			let { x, y } = action.payload;
			x += state.x;
			y += state.y;
			return { x, y };
	}
	return state;
};

const statsReducer = (state = initialState.stats, action) => {
  let { health, maxHealth } = state;
	
  switch (action.type) {
    case Actions.DRINK_POTION:
      health = Math.min(health + 20, maxHealth);
      return { ...state, health, maxHealth };
			
    case Actions.TAKE_DAMAGE:
      health = Math.max(0, health - action.payload);
      return { ...state, health };
  }
  return state;
};

const inventoryReducer = (state = initialState.inventory, action) => {
  let { potions } = state;
	
  switch (action.type) {
    case Actions.DRINK_POTION:
      potions = Math.max(0, potions - 1);
      return { ...state, potions };
  }
  return state;
};

//  Bootstrapping
const reducer = combineReducers({
	xp: xpReducer,
	level: levelReducer,
	position: positionReducer,
	stats: statsReducer,
	inventory: inventoryReducer,
});
const store = createStore(reducer);

store.subscribe(() => {
	console.log(JSON.stringify(store.getState()));
});

//  Run!
store.dispatch(move(1, 0));
store.dispatch(move(0, 1));
store.dispatch(takeDamage(13));
store.dispatch(drinkPotion());
store.dispatch(gainXp(100));
store.dispatch(levelUp());