Good question. I like to to have a ui part to my store. The reducer for which might look like this:

const initialState = {

    screenWidth: typeof window === 'object' ? window.innerWidth : null

};



function uiReducer(state = initialState, action) {

    switch (action.type) {

        case SCREEN_RESIZE:

            return Object.assign({}, state, {

                screenWidth: action.screenWidth

            });

    }

    return state;

}

The action for which is pretty boilerplate. (SCREEN_RESIZE being a constant string.)

function screenResize(width) {

    return {

        type: SCREEN_RESIZE,

        screenWidth: width

    };

}

Finally you wire it together with an event listener. I would put the following code in the place where you initialise your store variable.

window.addEventListener('resize', () => {

    store.dispatch(screenResize(window.innerWidth));

});

Media Queries

If your app takes a more binary view of screen size (e.g. large/small), you might prefer to use a media query instead. e.g.

const mediaQuery = window.matchMedia('(min-width: 650px)');



if (mediaQuery.matches) {

    store.dispatch(setLargeScreen());

} else {

    store.dispatch(setSmallScreen());

}



mediaQuery.addListener((mq) => {

    if (mq.matches) {

        store.dispatch(setLargeScreen());

    } else {

        store.dispatch(setSmallScreen());

    }

});

(I'll leave out the action and reducer code this time. It's fairly obvious what they look like.)

One drawback of this approach is that the store may be initialised with the wrong value, and we're relying on the media query to set the correct value after the store has been initialised. Short of shoving the media query into the reducer file itself, I don't know the best way around this. Feedback welcome.

UPDATE

Now that I think about it, you can probably get around this by doing something like the following. (But beware, I have not tested this.)

const mediaQuery = window.matchMedia('(min-width: 650px)');



const store = createStore(reducer, {

    ui: {

        largeScreen: mediaQuery.matches

    }

});



mediaQuery.addListener((mq) => {

    if (mq.matches) {

        store.dispatch(setLargeScreen());

    } else {

        store.dispatch(setSmallScreen());

    }

});

UPDATE II: The drawback of this last approach is that the ui object will replace the entire ui state not just the largeScreen field. Whatever else there is of the initial ui state gets lost.