Reducer.ts

export function apiKey(state: ApiKeyState, action: BaseAction) : ApiKeyState {
    let newState: ApiKeyState;

    switch (action.type) {
        case 'ADD_API_KEY':
            newState = Object.assign({}, state);
            newState.data.apiKeys = [
                ...state.data.apiKeys,
                action.payload.Key
            ] as string[];
            break;
        //More actions handling here
        //...
    }

    //ERROR: TS2454:Variable 'newState' is used before being assigned.
    if (newState) {
        return newState;
    } else {
        return state ? state : API_KEY_INITIAL_STATE;
    }
}