JAVASCRIPT

reducer react

const getTotalItems = (items: CartItemType[]) =>
    items.reduce((ack: number, item) => ack + item.amount, 0);

redux reducer example

import * as ActionTypes from './ActionTypes';

export const comments = (state = { errMess: null, comments: []}, action) => {
    switch (action.type) {
        case ActionTypes.ADD_COMMENTS:
            return {...state, errMess: null, comments: action.payload};

        case ActionTypes.COMMENTS_FAILED:
            return {...state, errMess: action.payload};

        default:
            return state;
    }
};