Array.Map() is changing state without using return value

I am coding along with the Redux Essentials tutorial.

When I run notificationArray.map without using the return value, my state is updated per the map functions callbacks.

I.e. in allNotifiicationsRead, each notifications's read property is set to true in the app state. And in fetchNotifications, all each notification's isNew property is set to !read.

I would expect this notificationArray.map(...) call to not have any effect since, I am not using the return value, let alone editing the state with it such as something like notificationsAdapter.setMany(notificationArray).

Any clarification on how this is working would be appreciated.

Please let me know if I didn't include any relevant details about this app.

const notificationsSlice = createSlice({
    name: 'notifications',
    initialState: notificationsAdapter.getInitialState(),
    reducers: {
        allNotificationsRead(state, action) {
            const notificationArray = Object.values(state.entities)
            notificationArray.map(notification => (
                notification.read = true
            ))
        }
    },
    extraReducers(builder){
        builder.addCase(fetchNotifications.fulfilled, (state, action) => {
            notificationsAdapter.upsertMany(state, action.payload)
            const notificationArray = Object.values(state.entities)
            notificationArray.map(notification => (
                notification.isNew = !notification.read
            ))
        })
    }
})