Angular2 chain http request

This answer is not specific to Angular, but mostly shows how to chain Reactive operators together to get what you need. You can do something like the following (warning: untested code):

import 'rxjs/add/operator/map';
import 'rxjs/add/operator/switch';

export class Foo {
    bar() {
        const httpClient: HttpClient = /* Your HTTP client here */;
        httpClient
            .get<UrlAndString[]>('first url')
            .map(urlsAndStrings => {
                const parameter = doSomethingWithUrlsAndStrings(urlsAndStrings);
                return httpClient
                    .get<SecondResult>(`second url?param=${parameter}`)
                    .map(secondResult => { return [urlsAndStrings, secondResult]; });
            })
            .switch()
            .subscribe(tuple => {
                console.log(tuple[0]); // The urls and strings, of type UrlAndString[].
                console.log(tuple[1]); // The second result, of type SecondResult.
            });
    }
}

The import statements bring in the map and switch operators so that you can chain them.

map transforms events as they come through, so you can see that when the first .get() returns something it will be mapped to another Observable<T>. So the type signature of the first map function will be map<UrlAndString[], Observable<[UrlAndString[], SecondResult]>>. Meaning it will return an Observable<Observable<[UrlAndString[], SecondResult]>>, or an Observable that publishes Observables which publish tuples of type UrlAndString[] and SecondResult.

switch takes an Observable<Observable<T>> and turns it into an Observable<T>. The documentation I linked to above explains it better than I can. But you can see that the return type of that .switch() will be an Observable<[UrlAndString[], SecondResult]>.

The .subscribe() you're probably already familiar with.