How to make a sequence of http requests in Angular 6 using RxJS

I've been looking for a solution all over the web, but couldn't find anything that fits my user case. I'm using the MEAN stack (Angular 6) and I have a registration form. I'm looking for a way to execute multiple HTTP calls to the API and each one is dependent on the returned result from the previous one. I need something that looks like this:

firstPOSTCallToAPI('url', data).pipe(
    result1 => secondPOSTCallToAPI('url', result1)
    result2 => thirdPOSTCallToAPI('url', result2)
    result3 => fourthPOSTCallToAPI('url', result3)
    ....
).subscribe(
    success => { /* display success msg */ },
    errorData => { /* display error msg */ }
);

What combination of RxJS operators do I need to use to achieve this? One possible solution would be to nest multiple subscriptions, but I want to avoid that and do it better with RxJS. Also need to think about error handling.

For calls that depend on previous result you should use concatMap

firstPOSTCallToAPI('url', data).pipe(
    concatMap(result1 => secondPOSTCallToAPI('url', result1))
      concatMap( result2 => thirdPOSTCallToAPI('url', result2))
       concatMap(result3 => fourthPOSTCallToAPI('url', result3))
    ....
).subscribe(
    success => { /* display success msg */ },
    errorData => { /* display error msg */ }
);

if your async method does not depend on return value of the precedent async call you can use

   concat(method(),method2(),method3()).subscribe(console.log)

I have faced same problem, this is my solution use pipe and concatMap for array for get sequence data for time period between start and end time.

This is general solution when we have array request.

I share for whom concern.

 let currentReplayData = [];
 let timerange = [[t1, t2], [t3, t4]]; // array of start and end time
 from(timerange).pipe(
      concatMap(time => <Observable<any>>this.dataService.getData(time[0],time[1]))
      ).subscribe(val => {
        //console.log(val)
        this.currentReplayData = this.currentReplayData.concat(val);
      });

MergeMap

is exact what you are looking for

firstPOSTCallToAPI('url', data).pipe(
    mergeMap(result1 => secondPOSTCallToAPI('url', result1)),
    mergeMap(result2 => thirdPOSTCallToAPI('url', result2)),
    mergeMap(result3 => fourthPOSTCallToAPI('url', result3)),
    // ...
).subscribe(
    success => { 
      // here you will get response of LAST request (fourthPOSTCallToAPI)
    },
    errorData => { /* display error msg */ }
);


// I assume that
// secondPOSTCallToAPI, thirdPOSTCallToAPI and fourthPOSTCallToAPI
// returns obserwable eg. return this.http.get<Book>(apiUrl);