Where in React / Redux / Saga to compare the received data with the store?

Question

React

I receive data from the server with Ajax - a list of news.
It is necessary to filter from the list only those news that are not yet available and add them.
Here is the Saga code:

function* getNewsUpdate({payload}) {
    try {
        const response = yield call(getLatestNews,payload.lang);
        yield put({type: "LOADED_NEWS", payload: {lang: payload.lang, loadedNews:  response}});
    } catch (err) {
        console.log(err);
    }
}

Here I have a list of downloaded news, but no storage access.
And the nearest place where I can compare the resulting list with the store is the reducer:

case 'LOADED_NEWS':
            let {lang, loadedNews} = action.payload;
            let newElements = filterNewItems(state[lang], loadedNews);
            return {...state,
                [lang]: [...state[lang], ...newElements]
            };

But there is a feeling that the filterNewItems function in the reducer is not appropriate at all. it's not a place to manipulate data. But how can I compare the received data with the state before the reducer - I'll never know.

Answer 1

In the reducer.