wp-calypso/client/state/countries/reducer.ts

import { withStorageKey } from '@automattic/state-utils';
import {
	COUNTRIES_DOMAINS_UPDATED,
	COUNTRIES_SMS_UPDATED,
	COUNTRIES_WOOCOMMERCE_UPDATED,
} from 'calypso/state/action-types';
import { combineReducers } from 'calypso/state/utils';
import type { CountriesState } from './types';
import type { Reducer } from 'react';
import type { AnyAction } from 'redux';

function createListReducer< TType extends Exclude< keyof CountriesState, 'woocommerce' > >(
	updatedActionType: string
): Reducer< CountriesState[ TType ] | undefined, AnyAction > {
	return ( state: undefined | CountriesState[ TType ] = [], action: AnyAction ) => {
		switch ( action.type ) {
			case updatedActionType:
				return action.countries;
			default:
				return state;
		}
	};
}

function createObjectReducer< TType extends Extract< keyof CountriesState, 'woocommerce' > >(
	updatedActionType: string
): Reducer< CountriesState[ TType ] | undefined, AnyAction > {
	return ( state: undefined | CountriesState[ TType ] = {}, action: AnyAction ) => {
		switch ( action.type ) {
			case updatedActionType:
				return action.countries;
			default:
				return state;
		}
	};
}

const combinedReducer = combineReducers( {
	domains: createListReducer< 'domains' >( COUNTRIES_DOMAINS_UPDATED ),
	sms: createListReducer< 'sms' >( COUNTRIES_SMS_UPDATED ),
	woocommerce: createObjectReducer< 'woocommerce' >( COUNTRIES_WOOCOMMERCE_UPDATED ),
} );

const countriesReducer = withStorageKey( 'countries', combinedReducer );
export default countriesReducer;