Typical approach to receiving a transformed object from an HTTP service

One of the services I provide includes a method with the following implementation:

public fetchCrawls(page: number): Observable<ICrawl[]>{
        return this._http.get(this._crawlsURL + page)
            .map((res: Response) => {
                return {
                    crawls: <ICrawl[]>res.json(),
                    headers: res.headers
                }
            })
            .catch(this.handleError);
    }

I chose to implement it this way instead

.map((res: Response) => <ICrawl[]>res.json())

This allows me in the consumer component to utilize the headers for pagination:

fetchCrawls(page: number): void {
        this._crawlsService.fetchCrawls(page)
            .subscribe(
                res => {
                    this.crawls = res.crawls;
                    this.totalItems = res.headers.get('X-Records');
                },
                error => this.errorMessage = <any>error);
    }

Although it functions correctly, both res.crawls and res.headers show as unresolved variables in WebStorm (red), yet the code still compiles without issues.

This leads me to believe my current approach may be incorrect. How can I avoid having unresolved variables in this scenario?

Answer №1

It appears that there is a mistake in the way you have defined your Observable. Instead of using Observable<ICrawl[]>, you should use the following structure:

interface ICrawlResponse {
    crawls: ICrawl[];
    headers: Headers;
}

public getCrawls(page: number): Observable<ICrawlResponse>{
        return this._http.get(this._crawlsURL + page)
            .map((res: Response) => {
                return {
                    crawls: <ICrawl[]>res.json(),
                    headers: res.headers
                }
            })
            .catch(this.handleError);
    }

Answer №2

When working with callbacks, it's important to clearly specify the expected object type:

fetchResults(pageNumber: number): void {
  this._resultService.fetchResults(pageNumber)
        .subscribe(
          response:{results:IResult[],metadata:Metadata} => { // <------
            (...)
          }
        );

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