Angular Http Interceptor with Caching

I have a simple caching interceptor and I want that my cache is immutable. But somehow it's not working. The original code is from the HttpClient doc https://angular.io/guide/http#intercepting-all-requests-or-responses

export class HttpCachingInterceptor implements HttpInterceptor {
private cache = new Map<string, HttpResponse<any>>();

intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
    // Before doing anything, it's important to only cache GET requests.
    // Skip this interceptor if the request method isn't GET.
    if (req.method !== 'GET') {
        return next.handle(req);
    }

    // First, check the cache to see if this request exists.
    const cachedResponse = this.cache.get(req.urlWithParams);
    if (cachedResponse && !req.headers.get('disable-cache')) {
        // A cached response exists. Serve it instead of forwarding
        // the request to the next handler.
        return Observable.of(cachedResponse.clone());
    }

    // No cached response exists. Go to the network, and cache
    // the response when it arrives.
    return next.handle(req).do(event => {
        // Remember, there may be other events besides just the response.
        if (event instanceof HttpResponse && !req.headers.get('disable-cache')) {
            // Update the cache.
            this.cache.set(req.urlWithParams, event.clone());
        }
    });
}

}

I have two components where data is accessed. First component retrieves the complete data without cache (first request) and modifies the data by

myHttpService.getCategories().map((root) => {
        root.categories.splice(0, 1);

The second component uses the same service myHttpService but the response is served from the cache and the data is mutated. I know I could use slice in that case but my cache should be immutable how can I achieve that without using additional libraries?