FIX: TS2454: Variable 'newState' is used before being assigned.

Reducer.ts

export function apiKey(state: ApiKeyState, action: BaseAction) : ApiKeyState {
    //Assigning variable here.
    let newState: ApiKeyState = state;

    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
        //...
    }

    //Using only newState in return.
    return newState ? newState : API_KEY_INITIAL_STATE;
}

You could even shorten line 18 to the following:

return newState || API_KEY_INITIAL_STATE;