Angular 2 - Overridding HTTP requests in a unit test

Question

I am using Angular 2.31. I want to setup a unit test for a service that calls an API. To do this I need to fake the HTTP response from the API call.

In the following example I have a thin ExampleService that extends APIService which in turn handles all the backend complexity. I would subscribe to ExampleService.exampleData$ to get the information from the API call. I need to somehow override the HTTP request during the unit test so I can inject fake data.

Pseudo-code:

class ExampleService extends APIService {
    private _dataForTheAPI: {} = { ...stuff... };

    constructor(private _http: Http) {
        super(_http, this._dataForTheAPI);
    }

    get exampleData$(): BehaviorSubject<any> {
        return this.data$;
    }
}

class APIService {
    private _apiUrl: string = 'http://somewhere';
    private _storage$: BehaviorSubject<any[]>;

    constructor(private _http: Http, private _dataForTheAPI: any) {
        this._getDataFromApi(_dataForTheAPI);
    }

    private _getDataFromApi(data: any): void {
        this._http
            .post(this._apiUrl, JSON.stringify(data))
            .map(response => {
                this._storage$.next(response.json());
            });
    }

    get data$(): BehaviorSubject<any> {
        return this._storage$;
    }
}