[RxJS] How To get the results of two HTTP requests made in sequence

switchMap can chain two HTTP requests together, creating one request based on the results of the first request.

But the result observable did not have the data of the first request, instead it only had access to the data of the second HTTP request.

If we would like to have both the data of the first HTTP request and deliver it together with the data of the second request, we could use a selector function (notice the second argument passed to switchMap):

sequentialRequests() {

const sequence$ = this.http.get<Course>(
            '/courses/-KgVwEBq5wbFnjj7O8Fp.json')
    .switchMap(course => {
        course.description+= ' - TEST ';
        return this.http.put('/courses/-KgVwEBq5wbFnjj7O8Fp.json', course)
    },
    (firstHTTPResult, secondHTTPResult)  => 
               [firstHTTPResult, secondHTTPResult]);

sequence$.subscribe(
    values => console.log("result observable ", values) 
);
    
}

Other example:

//emit immediately, then every 5s
const source = Rx.Observable.timer(0, 5000);
//switch to new inner observable when source emits, invoke project function and emit values
const example = source.switchMap(() => Rx.Observable.interval(2000), (outerValue, innerValue, outerIndex, innerIndex) => ({outerValue, innerValue, outerIndex, innerIndex}));
/*
    Output:
    {outerValue: 0, innerValue: 0, outerIndex: 0, innerIndex: 0}
    {outerValue: 0, innerValue: 1, outerIndex: 0, innerIndex: 1}
    {outerValue: 1, innerValue: 0, outerIndex: 1, innerIndex: 0}
    {outerValue: 1, innerValue: 1, outerIndex: 1, innerIndex: 1}
*/
const subscribe = example.subscribe(val => console.log(val));