{
    type: 'ADD_ITEM_TO_CART',
    data: {
        id: 7337,
        name: 'OnePlus 6',
        price: 34999
    }
}

const initialState = {
    items: [],
    amount: 0,
};

function(state = initialState, action) {
    let newState;

    if (action.type === 'ADD_ITEM_TO_CART') {
        newState = {
            ...state,
            items: [...state.items, action.data],
            amount: state.amount + action.data.amount,
        };
    } else {
        newState = state;
    }

    return newState;
}

import { createStore } from 'redux';


function reducer(state = {}, action) {
    ...
}


const store = createStore(reducer);

const store = createStore(reducer);


store.getState();
/*
    This method returns the state of
    the store.
*/


store.dispatch(); 
/*
    This method allows you to dispatch
    `actions` to the store. Takes an
    `action` as a parameter.
*/

store.subscribe();
/*
    This method allows you to add
    listeners that trigger on store
    updates. Takes a callback as a 
    parameter.
*/

function addItem(item) {
    return {
        type: 'ADD_ITEM_TO_CART',
        data: item,
    }
}

import { combineReducers } from 'redux';

function cartReducer(state = {},  action) {
    ...
}

function productReducer(state = {},  action) {
    ...
}


const rootReducer = combineReducer({
    cart: cartReducer,
    product: productReducer,
});

/*
This would generate the following state object

{
    cart: {
        // all state values managed by cartReducer
    },
    product: {
        // all state values managed by productReducer
    }
}
*/

const store = createStore(rootReducer);